Switching power supply apparatus

ABSTRACT

A switching power supply apparatus operates with less power consumption as a whole as a result of reduced power loss suffered while the switching operation of the main switching device is being stopped in burst switching control. Burst switching control is achieved by a signal level checker circuit  15  repeatedly turning on and off a switch circuit  17  provided in the line by way of which a switching controller circuit  19  is supplied with operating power. In burst switching control, when the switching operation of a main switching device  5  is being stopped, the supply of operating power to the switching controller circuit  19  is also stopped. This helps reduce the power loss suffered while the switching operation is being stopped, and thus helps reduce the power consumption of the apparatus as a whole.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a switching power supplyapparatus used as a direct-current power source in an electronicappliance.

[0003] 2. Description of the Prior Art

[0004] A conventionally known example of such a switching power supplyapparatus is disclosed, for example, in Japanese Patent ApplicationLaid-Open No. H10-304658. The switching power supply apparatus disclosedin this publication is provided with a main switch that turns on and offa direct current applied to the primary coil of a transformer, asecondary-side rectifying/smoothing circuit that rectifies and smoothesthe on/off signal induced in the secondary coil of the transformer so asto supply it as a main output signal, a subsidiary power source thatrectifies and smoothes the on/off signal induced in the bias coil of thetransformer so as to supply it as a subsidiary supply voltage, an erroramplifier that generates an error voltage signal that represents thedifference between the subsidiary supply voltage output from thesubsidiary power source and a reference voltage, and a comparator thatfeeds the main switch with an on/off control signal in such a way as toreduce the error voltage signal output from the error amplifier. Thisswitching power supply apparatus is further provided with a light-loadswitching controller portion that temporarily stops the on/off operationof the main switch when the main output voltage becomes higher than anupper limit voltage and that restarts the on/off operation of the mainswitch when the main output voltage becomes lower than a lower limitvoltage.

[0005] In this conventional switching power supply apparatus, control isso performed that the on/off operation of the main switch is temporarilystopped when the main output voltage output from the secondary-siderectifying/smoothing circuit becomes higher than the upper limitvoltage, and that the on/off operation of the main switch is restartedwhen the main output voltage becomes lower than the lower limit voltage.

[0006] Here, however, while the on/off operation of the main switch istemporarily stopped when the main output voltage becomes higher than theupper limit voltage, operating power is kept supplied to the individualcircuits and control devices provided in the control circuit that drivesand controls the main switch. This causes wasteful power loss.Specifically, in a configuration where the on/off operation of the mainswitch is temporarily stopped when the main output voltage output fromthe secondary-side rectifying/smoothing circuit becomes higher than theupper limit voltage and the on/off operation of the main switch isrestarted when the main output voltage becomes lower than the lowerlimit voltage, i.e., in so-called burst switching control, the operatingpower is kept supplied to all the circuits and control devices providedin the control circuit even while the switching operation is beingstopped. This causes wasteful consumption of the supply current,resulting in wasteful power loss.

[0007] Also in the conventional switching power supply apparatusesdisclosed in Japanese Patent Applications Laid-Open Nos. 2001-346378 and2002-58238, as in the switching power supply apparatus described above,even during the period in which the switching operation of the mainswitch is being stopped in burst switching control, the operating poweris kept supplied to all the circuits and control devices provided in theswitching signal controlling circuit. This causes wasteful consumptionof the supply current, resulting in wasteful power loss.

[0008] Incidentally, in the conventional switching power supplyapparatus disclosed in Japanese Patent Application Laid-Open No.2001-86745, to reduce power consumption in a stand-by state, theswitching operation of the main switching device is stopped in thatstate. Thus, the aim of this invention is not to reduce the power losssuffered while the switching operation of the main switching device isbeing stopped in burst switching control.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide a switchingpower supply apparatus that operates with less power consumption as awhole as a result of reduced power loss suffered while the switchingoperation of the main switching device is being stopped in burstswitching control.

[0010] To achieve the above object, according to one aspect of thepresent invention, a switching power supply apparatus has a serialcircuit, including the primary coil of a transformer and a mainswitching device, connected between a positive and a negative powersupply line connected to a direct-current power source. The switchingpower supply apparatus outputs a direct-current voltage obtained byrectifying with a rectifier a high-frequency voltage induced in thesecondary coil of the transformer by the main switching deviceperforming switching operation. Here, the switching power supplyapparatus uses as a feedback signal the result of comparison between thedirect-current voltage and a predetermined reference voltage, and drivesthe main switching device by turning on and off, according to the signallevel of the feedback signal, supply of operating power to a mainswitching device driving system that drives the main switching device.

[0011] In this switching power supply apparatus according to theinvention, for example, in heavy-load operation, the output voltagedecreases. To correct this, a lower-level feedback signal is generated.This causes the operating power to the main switching device drivingsystem to be kept supplied thereto, and thus the main switching devicecontinues switching operation. On the other hand, in light-loadoperation, when the output voltage becomes higher than a predeterminedvalue, a higher-level feedback signal is generated. This causes thesupply of the operating power to the switching device driving system tobe stopped, and thus the main switching device stops switchingoperation. As a result, the output voltage returns to the predeterminedvalue.

[0012] That is, with this switching power supply apparatus according tothe invention, while the switching operation of the main switchingdevice is being stopped in burst switching control, the supply of theoperating power to the main switching device driving system is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0013] According to another aspect of the present invention, a switchingpower supply apparatus has a serial circuit, including the primary coilof a transformer and a main switching device, connected between apositive and a negative power supply line connected to a direct-currentpower source. The switching power supply apparatus outputs adirect-current voltage obtained by rectifying with a rectifier ahigh-frequency voltage induced in the secondary coil of the transformerby the main switching device performing switching operation. Here, theswitching power supply apparatus further includes: an output voltagedetector that compares the direct-current voltage obtained throughrectification with a predetermined reference voltage and that outputsthe result of the comparison as a feedback signal; a switchingcontroller that drives and controls the main switching device accordingto the feedback signal output from the output voltage detector; a signallevel checker that monitors the signal level of the feedback signal andthat outputs an operation control signal for turning on and off theswitching controller according to the monitored signal level; and anoperation/nonoperation switcher that is provided in the line by way ofwhich the switching controller is supplied with operating power and thatturns on and off the switching controller according to the operationcontrol signal from the signal level checker.

[0014] In this switching power supply apparatus according to theinvention, in light-load operation, when the output voltage tends toincrease, i.e., when the output voltage is higher than a predeterminedvalue, and thus the signal level of the feedback signal output from theoutput voltage detector is, for example, high, the signal level checkerfeeds the operation/nonoperation switcher with an operation controlsignal that requests nonoperation, and thus the operation/nonoperationswitcher stops the supply of the operating power to the switchingcontroller.

[0015] As a result, the main switching device stops switching operation,and thus the output voltage starts to decrease gradually. When thesignal level of the feedback signal from the output voltage detectorbecomes, for example, low, the signal level checker feeds theoperation/nonoperation switcher with an operation control signal thatrequests operation, and thus the operation/nonoperation switcher startsthe supply of the operating power to the switching controller.

[0016] As a result, the main switching device restarts switchingoperation, and thus the output voltage starts to increase gradually.When the signal level of the feedback signal becomes high again, thesignal level checker feeds the operation/nonoperation switcher with anoperation control signal that requests nonoperation, and thus theoperation/nonoperation switcher stops the supply of the operating powerto the switching controller. As a result, the main switching devicestops switching operation, and thus the output voltage starts todecrease gradually. As this sequence of operations is repeated, theoutput voltage is kept at the predetermined value.

[0017] In this switching power supply apparatus, when the output voltagetends to decrease, i.e., when the output voltage is lower than apredetermined value, and thus the signal level of the feedback signaloutput from the output voltage detector is, for example, low, the signallevel checker feeds the operation/nonoperation switcher with anoperation control signal that requests operation, and thus theoperation/nonoperation switcher continues supplying the operating powerto the switching controller so that switching operation is performedcontinuously.

[0018] With this switching power supply apparatus according to theinvention, burst switching control is achieved as a result of the signallevel checker repeatedly turning on and off the operation/nonoperationswitcher provided in the line by way of which the switching controlleris supplied with operating power. Moreover, while the switchingoperation of the main switching device is being stopped in burstswitching control, the supply of the operating power to the switchingcontroller is also stopped. This helps reduce the power loss sufferedwhile the switching operation is being stopped, and thus helps reducethe power consumption of the apparatus as a whole.

[0019] Preferably, the feedback signal from the output voltage detectoris transmitted to the switching controller through the photodiode of aphotocoupler, and the signal level checker monitors the signal level ofthe feedback signal by comparing the current level flowing through thephototransistor of the photocoupler with a reference current level.

[0020] With this configuration, burst switching operation is controlledaccording to the result of comparison between the current value throughthe phototransistor and the reference current value. The signal level ofthe feedback signal (i.e., the current value through thephototransistor) represents the load current value of the switchingpower supply apparatus. Thus, it is possible to correctly set the loadcurrent value at which switching between continuous switching operationand burst switching operation is performed.

[0021] In burst switching operation, the output voltage fluctuates.However, since the signal level of the feedback signal represents theoutput voltage value, it is possible to correctly set the upper andlower limits of the output voltage.

[0022] Preferably, a current detection resistor is connected in serieswith the phototransistor of the photocoupler, and the signal levelchecker turns on and off the switching controller by feeding theswitching controller with, as the operation control signal, a signalobtained by comparing the voltage drop across the current detectionresistor with the voltage of a current level check reference powersource.

[0023] With this configuration, it is possible to make the signal levelchecker detect the signal level of the feedback signal according to thevoltage drop across the current detection resistor, compare the signallevel with the voltage of the current level check reference powersource, and turn on and off the supply of the operating power to theswitching controller according to the result of the comparison.

[0024] Preferably, the operating power of the switching controller issupplied by way of the start-up current supply line by way of which astart-up current is supplied from the positive power supply line througha start-up resistor, or by way of the steady-operation current supplyline by way of which a voltage induced in the subsidiary coil of thetransformer is supplied after being rectified with a serial circuitcomposed of a plurality of diodes, and the operating power of the signallevel checker is supplied from subsidiary control power extracted from anode between the plurality of diodes.

[0025] With this configuration, when the switching power supplyapparatus starts to start up, it is possible to prevent, by the actionof the diodes, the current that is supposed to flow to the start-upcurrent supply line from flowing to the steady-operation current supplyline. This helps reduce the time required for start-up, and also helpsreduce the resistance of the start-up resistor and thereby reduce powerconsumption.

[0026] In other words, as compared with a switching power supplyapparatus that is not provided with the function of controlling burstswitching in such a way as to stop the supply of the operating power tothe switching controller that performs burst switching operation whenthe switching power supply apparatus starts to start up, the switchingpower supply apparatus of the present invention starts up in as short atime while reducing the unnecessary power consumption by the start-upresistor.

[0027] Preferably, the operating power of the signal level checker andthe phototransistor of the photocoupler is supplied from subsidiarycontrol power extracted from a node between a plurality of diodesconstituting a serial circuit provided in the steady-operation currentsupply line by way of which a voltage induced in the subsidiary coil ofthe transformer is supplied after being rectified with the plurality ofdiodes.

[0028] With this configuration, when the switching power supplyapparatus starts to start up, the diodes prevent the start-up currentfrom flowing to the subsidiary control power. This helps shorten thestart-up time. Moreover, in the steady operation, the direct-currentvoltage obtained by rectifying the voltage induced in the subsidiarycoil of the transformer is fed as the operating power to the signallevel checker and the phototransistor of the photocoupler. This ensuresstable operation.

[0029] Preferably, the switching controller is realized as a PWM controlcircuit that outputs, as the drive signal with which to drive the mainswitching device, a pulse signal that is pulse-width-modulated accordingto the voltage level of the feedback signal from the output voltagedetector.

[0030] With this configuration, the main switching device is driven witha drive signal accurately commensurate with the voltage level of thefeedback signal. This helps enhance the stability of the output voltageof the switching power supply apparatus.

[0031] Preferably, used as the PWM control circuit is a PWM control IC(for example, an IC with the product number FA5511 manufactured by FujiElectric Co., Ltd.) that is realized as an integrated circuit chiphaving at least an FB terminal to which a voltage related to thefeedback signal is input and a CS terminal to which a voltage forenabling or disabling an internal circuit is input.

[0032] With this configuration, it is possible to reduce the spaceoccupied by the circuit that drives the main switching device, and toenhance the stability of the output voltage, leading to miniaturizationof the apparatus.

[0033] Preferably, when a PWM control IC is used as the switchingcontroller, a start-up corrector is additionally provided to correct thestart-up of the PWM control IC; a first resistor is connected betweenthe FB terminal of the PWM control IC and the negative power supplyline; the signal level checker feeds a CS terminal controller, whichserves as the operation/nonoperation switcher, and the FB terminal withthe operation control signal and an inverted feedback signal,respectively, according to the result of checking of the signal level ofthe feedback signal; the CS terminal controller connects and disconnectsthe CS terminal of the PWM control IC to and from the negative powersupply line according to the operation control signal; and the start-upcorrector connects and disconnects, through a second resistor, the FBterminal to and from the negative power supply line according to thevoltage level of the subsidiary control power.

[0034] With this configuration, at the start-up of the switching powersupply apparatus, when the voltage of the subsidiary control powerincreases, immediately before a current starts to flow through thephototransistor, the start-up corrector connects the second resistor inparallel with the first resistor and thereby reduces the resistancebetween the FB terminal and the negative power supply line. This causesthe potential at the FB terminal to decrease. In this way, when theswitching power supply apparatus starts to start up, the voltage at theFB terminal is kept at the optimum level to permit reliable rising ofthe output voltage. In addition, in the steady state, the switchingpower supply apparatus is permitted to output a reliably stabilizedvoltage.

[0035] Preferably, the signal level checker includes a pair oftransistors having the emitters thereof connected together to form acomparator, with the base of one of the transistors connected to thenode between the current detection resistor and the phototransistor,with the base of the other of the transistors connected to the currentlevel check reference power source, with the collector of the one of thetransistors connected to the FB terminal of the PWM control IC, and withthe collector of the other of the transistors connected to the CSterminal controller.

[0036] With this configuration, it is possible to easily realize thecomparator for comparing the signal level of the feedback signal withthe current level of the current level check reference power.

[0037] Preferably, the CS terminal controller includes an NPN-typetransistor having the collector thereof connected to the CS terminal ofthe PWM control IC, having the emitter thereof connected to the negativepower supply line, and having the base thereof connected to thecollector of the other of the transistors included in the signal levelchecker.

[0038] With this configuration, where the CS terminal controller isprovided with the NPN transistor connected in the manner describedabove, it is possible, with a simple configuration, to enable anddisable the PWM control IC.

[0039] Preferably, the start-up corrector includes: a serial circuitcomposed of a Zener diode and a plurality of resistors connected betweenthe line of the subsidiary control power and the negative power supplyline; and an NPN-type transistor having the base thereof connected to anode between the resistors, having the collector thereof connectedthrough the second resistor to the FB terminal of the PWM control IC,and having the emitter thereof connected to the negative supply powerline.

[0040] With this configuration, where the start-up corrector is providedwith the serial circuit and the NPN-type transistor described above, itis possible, with a simple configuration, to make the switching powersupply apparatus output a reliably stabilized voltage in the steadyoperation.

[0041] Preferably, the signal level checker includes, for generation ofthe reference voltage, voltage division resistors, of which alower-potential-side resistor is divided into two resistors, with thenode therebetween connected through a diode to the CS terminal of thePWM control IC.

[0042] With this configuration, by varying the resistances of theindividual division resistors for generating the reference voltage, itis possible to freely and accurately set the fluctuation width and burstswitching period of the output voltage in burst switching operation. Inparticular, by making the fluctuation width of the output voltage aswide as applications permit, it is possible to reduce unnecessary powerconsumption in burst switching operation.

[0043] Preferably, the switching power supply apparatus furtherincludes: a capacitor connected between the CS terminal of the PWMcontrol IC and the negative power supply line; and a diode connectedbetween the capacitor and the CS terminal.

[0044] With this configuration, in burst switching operation, it ispossible to quicken, by the action of the diode, the fluctuation of thevoltage level at the CS terminal and thereby quicken the speed ofswitching between a state in which switching operation is performed anda state in which switching operation is stopped. Moreover, in burstswitching operation, it is possible to reduce the fluctuation width ofthe output voltage and increase the accuracy of the upper and lowerlimits of the output voltage. Moreover, when the load current abruptlyincreases during burst switching operation, it is possible to shortenthe time required to shift to continuous switching operation and therebyprevent a decrease in the output voltage.

[0045] Preferably, when a PWM control IC is used as the switchingcontroller, the switching power supply apparatus further includes: acurrent adjuster connected between the FB terminal of the PWM control ICand the negative power supply line to adjust the current output from theFB terminal according to the signal level of the feedback signal; and aCS terminal controller that serves as the operation/nonoperationswitcher by connecting and disconnecting the CS terminal of the PWMcontrol IC to and from the negative power supply line according to anoutput signal of the signal level checker.

[0046] With this configuration, at the start-up of the switching powersupply apparatus, the current adjuster adjusts the voltage at the FBterminal of the PWM control IC to a high value. Thus, the PWM control ICmakes the main switching device perform switching operation with a greaton-state duty, and thereby reduces the start-up time. Moreover, the CSterminal controller connects and disconnects the CS terminal of the PWMcontrol IC to and from the negative power supply line according to theoutput signal of the signal level checker, and thereby turns on and offthe PWM control IC.

[0047] Preferably, the current adjuster includes an NPN-type transistorhaving the collector thereof connected to the FB terminal of the PWMcontrol IC, having the emitter thereof connected through a resistor tothe negative power supply line, and having the base thereof connected tothe line of the feedback signal.

[0048] With this configuration, the current adjuster has a simplerconfiguration than the start-up corrector, but nevertheless achieves thesame effect. That is, it is possible, with a simpler configuration, toreduce the start-up time of the switching power supply apparatus.

[0049] According to another aspect of the present invention, theswitching power supply apparatus has a serial circuit, including theprimary coil of a transformer and a main switching device, connectedbetween a positive and a negative power supply line connected to adirect-current power source. The switching power supply apparatusoutputs a desired direct-current voltage by controlling the mainswitching device according to a feedback signal obtained as a result ofcomparison between a direct-current voltage obtained throughrectification of a high-frequency voltage induced in the secondary coilof the transformer by the main switching device performing switchingoperation and a previously set reference voltage. Here, the signal levelof the feedback signal is compared with the signal level of a previouslygenerated oscillation signal. According to the result of the comparison,the on-state duty of the drive signal to be fed to the main switchingdevice is determined and switching between burst switching control andcontinuous switching control is performed. Moreover, while the switchingoperation of the main switching device is being stopped in burstswitching control, supply of the operating power for driving the mainswitching device is stopped.

[0050] In this switching power supply apparatus according to theinvention, the on-state duty of the drive signal to be fed to the mainswitching device is determined according to the result of comparisonbetween the signal level of the previously generated oscillation signaland the signal level of the feedback signal. This makes it possible toaccurately control the switching of the main switching device. Moreover,switching between burst switching and continuous switching is alsoperformed according to the result of the comparison. This makes itpossible to accurately perform the switching. Moreover, while theswitching operation of the main switching device is being stopped, thesupply of the operating power for driving the main switching device isstopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0051] Preferably, burst switching control is achieved by turning on andoff the supply of operating power to the switching controller thatdrives the main switching device. This helps reduce the power losssuffered while the switching operation is being stopped.

[0052] Preferably, when a PWM control IC is used as the switchingcontroller, a capacitor is connected between the FB terminal of the PWMcontrol IC and an internal power terminal connected to an internal powersupply line.

[0053] With this configuration, in a case where, for phase compensationof the output voltage stabilizing control system, a serial circuitcomposed of a capacitor and a resistor is connected between the FBterminal of the PWM control IC and the negative power supply line, evenwhen the load current abruptly increases during burst switchingoperation, it is possible to quicken the control of the burst switchingoperation control system as much as possible and thereby prevent adecrease in the output voltage of the switching power supply apparatus.In addition, it is possible to reduce unnecessary power consumption inthe burst switching operation.

[0054] Preferably, when a PWM control IC is used as the switchingcontroller, a serial circuit composed of a capacitor and a resistor isconnected between the FB terminal of the PWM control IC and an internalpower terminal connected to an internal power supply line.

[0055] With this configuration, in a case where, for phase compensationof the output voltage stabilizing control system, a serial circuitcomposed of a capacitor and a resistor is connected between the FBterminal of the PWM control IC and the negative power supply line, evenwhen the load current abruptly increases during burst switchingoperation, it is possible to quicken the control of the burst switchingoperation control system as much as possible and thereby prevent adecrease in the output voltage of the switching power supply apparatus.In addition, it is possible to achieve phase compensation in the outputvoltage stabilizing control system with almost no effects on the burstswitching operation characteristics.

[0056] Preferably, the current adjuster includes an NPN-type transistorhaving the collector thereof connected to the FB terminal of the PWMcontrol IC, having the emitter thereof connected through a resistor tothe negative power supply line, and having the base thereof connected tothe line of the feedback signal, and in series with the resistorconnected between the base of the NPN-type transistor and the negativepower supply line is connected an NPN-type transistor having thecollector and base thereof connected together.

[0057] With this configuration, even when characteristics change astemperature varies, the current adjuster can suppress the variation ofthe predetermined current value (of the load current) at which switchingbetween burst switching operation and continuous switching operation isperformed. This helps stabilize the output voltage.

[0058] Preferably, when a PWM control IC is used as the switchingcontroller, a start-up corrector is additionally provided to correct thestart-up of the PWM control IC; a start-up switcher is additionallyprovided to turn on and off the supply of operating power to the signallevel checker; a first resistor is connected between the FB terminal ofthe PWM control IC and the negative power supply line; the signal levelchecker feeds a CS terminal controller, which serves as theoperation/nonoperation switcher, and the FB terminal with the operationcontrol signal and an inverted feedback signal, respectively, accordingto the result of checking of the signal level of the feedback signal;the CS terminal controller connects and disconnects the CS terminal ofthe PWM control IC to and from the negative power supply line accordingto the operation control signal; the start-up corrector detects whetheror not the feedback signal is present so that, if the feedback signal ispresent, the start-up corrector connects, through a second resistor, theFB terminal of the PWM control IC to the negative power supply line and,if not, the start-up corrector cuts off the second resistor; and thestart-up switcher detects whether or not the feedback signal is presentso that, if the feedback signal is present, the start-up switcher turnson the supply of the operating power to the signal level checker and, ifnot, the start-up switcher turns off the supply of the operating powerto the signal level checker.

[0059] With this configuration, at the start of start-up, powerimmediately starts to be supplied to the PWM control IC, and theswitching power supply apparatus starts switching operation. Thisswitching operation causes the output voltage of the switching powersupply apparatus to increase until a feedback signal is generated, whenthe feedback signal is detected by the start-up corrector and thestart-up switcher. As a result, the start-up corrector connects thesecond resistor in addition to and in parallel with the first resistor,and the start-up switcher starts to supply an operating current to thesignal level checker. Supplied with the operating current, the signallevel checker starts to operate, and, during the period in which thesignal level of the feedback signal is lower than the voltage level ofthe current level check reference power, the CS terminal controllerkeeps the CS terminal of the PWM control IC disconnected from thenegative power supply line so that operating power is kept supplied tothe PWM control IC. Thus, switching operation is continued to permit theoutput voltage of the switching power supply apparatus to increase to apredetermined value.

[0060] Thereafter, when the load of the switching power supply apparatusis light, and the signal level of the feedback signal is found to behigher than the voltage level of the current level check referencepower, the CS terminal controller connects the CS terminal of the PWMcontrol IC to the negative power supply line to turn off the supply ofoperating power to the PWM control IC and thereby stop the switchingoperation of the switching power supply apparatus. As the output voltagedecreases, the signal level of the feedback signal decreases until itbecomes lower than the current level check reference, when the signallevel checker turns the operation control signal low. This causes the CSterminal controller to disconnect the CS terminal of the PWM control ICfrom the negative power supply line so that operating power is suppliedto the PWM control IC. This sequence of operations is repeated toachieve burst oscillation operation.

[0061] On the other hand, when the load of the switching power supplyapparatus is heavy, and the signal level of the feedback signal does notreach the voltage level of the current level check reference power, thesignal level checker turns the operation control signal low. This causesthe CS terminal controller to disconnect the CS terminal of the PWMcontrol IC from the negative power supply line so that continuousswitching operation is continued.

[0062] In particular, the start-up corrector is so configured as to cutoff the second resistor at start-up to increase the resistance betweenthe FB terminal of the PWM control IC and the negative power supply lineand thereby make the potential at the FB terminal higher. This ensuresreliable start-up operation. On the other hand, in the steady operation,the start-up corrector connects the second resistor in parallel with thefirst resistor to make the potential at the FB terminal of the PWMcontrol IC lower. This permits the PWM control IC to reliably controlthe switching power supply apparatus to output a stabilized voltage.

[0063] Preferably, the start-up switcher includes an NPN-type transistorhaving the collector thereof connected to the node between a currentdetection resistor connected to the line of the feed back signal and theinternal reference voltage line of the signal level checker, having thebase thereof connected to the phototransistor, and having the emitterthereof connected to the negative power supply line.

[0064] With this configuration, it is possible to realize the start-upswitcher with a simple circuit.

[0065] Preferably, the start-up corrector includes an NPN-typetransistor having the collector thereof connected through the secondresistor to the FB terminal of the PWM control IC, having the basethereof connected through a resistor to the phototransistor, and havingthe emitter thereof connected to the negative power supply line.

[0066] With this configuration, it is possible to realize the start-upcorrector with a simple circuit.

[0067] Preferably, when a PWM control IC is used as the switchingcontroller, a start-up corrector is additionally provided to correct thestart-up of the PWM control IC; a first resistor is connected betweenthe FB terminal of the PWM control IC and the negative power supplyline; the signal level checker feeds a CS terminal controller, whichserves as the operation/nonoperation switcher, and the FB terminal withthe operation control signal and an inverted feedback signal,respectively, according to the result of checking of the signal level ofthe feedback signal; the CS terminal controller connects and disconnectsthe CS terminal of the PWM control IC to and from the negative powersupply line according to the operation control signal; and the start-upcorrector detects whether or not the feedback signal is present so that,if the feedback signal is present, the start-up corrector connects,through a diode and the second resistor, the FB terminal of the PWMcontrol IC to the negative power source line and turns on the supply ofoperating power to the signal level checker and, if not, the start-upcorrector cuts off the diode and the second resistor and turns off thesupply of the operating power to the signal level checker.

[0068] With this configuration, at the start of start-up, powerimmediately starts to be supplied to the PWM control IC, and theswitching power supply apparatus starts switching operation. Thisswitching operation causes the output voltage of the switching powersupply apparatus to increase until a feedback signal is generated, whenthe feedback signal is detected by the start-up corrector. As a result,the start-up corrector connects the second resistor in addition to andin parallel with the first resistor, and starts to supply an operatingcurrent to the signal level checker. Supplied with the operatingcurrent, the signal level checker starts to operate, and, during theperiod in which the signal level of the feedback signal is lower thanthe voltage level of the current level check reference power, the CSterminal controller keeps the CS terminal of the PWM control ICdisconnected from the negative power supply line so that operating poweris kept supplied to the PWM control IC. Thus, switching operation iscontinued to permit the output voltage of the switching power supplyapparatus to increase to a predetermined value.

[0069] Thereafter, when the load of the switching power supply apparatusis light, and the signal level of the feedback signal is found to behigher than the voltage level of the current level check referencepower, the CS terminal controller connects the CS terminal of the PWMcontrol IC to the negative power supply line to turn off the supply ofoperating power to the PWM control IC and thereby stop the switchingoperation of the switching power supply apparatus. As the output voltagedecreases, the signal level of the feedback signal decreases until itbecomes lower than the current level check reference, when the signallevel checker turns the operation control signal low. This causes the CSterminal controller to disconnect the CS terminal of the PWM control ICfrom the negative power supply line so that operating power is suppliedto the PWM control IC. This sequence of operations is repeated toachieve burst oscillation operation.

[0070] On the other hand, when the load of the switching power supplyapparatus is heavy, and the signal level of the feedback signal does notreach the voltage level of the current level check reference power, thesignal level checker turns the operation control signal low. This causesthe CS terminal controller to disconnect the CS terminal of the PWMcontrol IC from the negative power supply line so that continuousswitching operation is continued.

[0071] In particular, the start-up corrector is so configured as to cutoff the second resistor at start-up to increase the resistance betweenthe FB terminal of the PWM control IC and the negative power supply lineand thereby make the potential at the FB terminal higher. This ensuresreliable start-up operation. On the other hand, in the steady operation,the start-up corrector connects the second resistor in parallel with thefirst resistor to make the potential at the FB terminal of the PWMcontrol IC lower. This permits the PWM control IC to reliably controlthe switching power supply apparatus to output a stabilized voltage.

[0072] The diode prevents a current from flowing through the signallevel checker in a predetermined timing period, and thereby prevents thesignal level checker from operating unnecessarily, contributing tohigher operation accuracy.

[0073] Preferably, the start-up corrector includes an NPN-typetransistor having the collector thereof connected through the diode andthe second resistor to the FB terminal of the PWM control IC, having thebase thereof connected through a resistor to the phototransistor, andhaving the emitter thereof connected to the negative power supply line.

[0074] With this configuration, it is possible to realize the start-upcorrector with a simple circuit.

[0075] Preferably, the signal level checker includes, for generation ofthe reference voltage, voltage division resistors, of which alower-potential-side resistor is divided into two resistors, with thenode therebetween connected through a diode to the CS terminalcontroller, and the CS terminal controller is connected through anotherdiode to the CS terminal of the PWM control IC.

[0076] With this configuration, by varying the resistances of theindividual division resistors for generating the reference voltage, itis possible to freely and accurately set the fluctuation width and burstswitching period of the output voltage in burst switching operation. Inparticular, by making the fluctuation width of the output voltage aswide as applications permit, it is possible to reduce unnecessary powerconsumption in burst switching operation.

[0077] Moreover, the other diode prevents a high-level voltage frombeing applied to the CS terminal of the PWM control IC when theswitching power supply apparatus starts to start up. This is because,when a high-level voltage is applied to the CS terminal, the output ofthe PWM control IC is turned off.

[0078] Preferably, when a PWM control IC is used as the switchingcontroller, a start-up switcher is additionally provided to turn on andoff the supply of operating power to the signal level checker; a currentadjuster is additionally provided that is connected between the FBterminal of the PWM control IC and the negative power supply line toadjust the current output from the FB terminal according to the signallevel of the feedback signal; the signal level checker feeds a CSterminal controller, which serves as the operation/nonoperationswitcher, with the operation control signal according to the result ofchecking of the signal level of the feedback signal; the CS terminalcontroller connects and disconnects the CS terminal of the PWM controlIC to and from the negative power supply line according to the operationcontrol signal; and the start-up switcher detects whether or not thefeedback signal is present so that, if the feedback signal is present,the start-up switcher turns on the supply of operating power to thesignal level checker and, if not, the start-up switcher turns off thesupply of operating power to the signal level checker.

[0079] With this configuration, at the start-up of the switching powersupply apparatus, the current adjuster adjusts the current at the FBterminal of the PWM control IC. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty,and thereby reduces the start-up time. Moreover, on detecting thefeedback signal, the start-up switcher starts to supply operating powerto the signal level checker. Moreover, the CS terminal controllerconnects and disconnects the CS terminal of the PWM control IC to andfrom the negative power supply line according to the output signal ofthe signal level checker, and thereby turns on and off the PWM controlIC. In this way, it is possible to realize burst switching operationwith high power use efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] This and other objects and features of the present invention willbecome clear from the following description, taken in conjunction withthe preferred embodiments with reference to the accompanying drawings inwhich:

[0081]FIG. 1 is a circuit diagram of the switching power supplyapparatus of a first embodiment of the invention;

[0082]FIG. 2 is a circuit diagram of the switching power supplyapparatus of a second embodiment of the invention;

[0083]FIG. 3 is a circuit diagram of the switching power supplyapparatus of a third embodiment of the invention;

[0084]FIG. 4 is a circuit diagram of the switching power supplyapparatus of a fourth embodiment of the invention;

[0085]FIG. 5 is a circuit diagram of a typical circuit configuration ofa switching power supply apparatus employing FA5511, for referencepurposes;

[0086]FIG. 6 is a circuit diagram showing an outline of the circuitconfiguration of FA5511;

[0087]FIG. 7 is a signal waveform diagram illustrating the start-upoperation of the switching power supply apparatus shown in FIG. 5;

[0088]FIG. 8 is a signal waveform diagram illustrating the start-upoperation of the switching power supply apparatus shown in FIG. 4;

[0089]FIG. 9 is a circuit diagram of the switching power supplyapparatus of a fifth embodiment of the invention;

[0090]FIG. 10 is a circuit diagram of the switching power supplyapparatus of a sixth embodiment of the invention;

[0091]FIG. 11 is a circuit diagram of the switching power supplyapparatus of a seventh embodiment of the invention;

[0092]FIG. 12 is a circuit diagram of the switching power supplyapparatus of an eighth embodiment of the invention;

[0093]FIG. 13 is a circuit diagram of the switching power supplyapparatus of a ninth embodiment of the invention;

[0094]FIG. 14 is a circuit diagram of the switching power supplyapparatus of a tenth embodiment of the invention;

[0095]FIG. 15 is a circuit diagram of the switching power supplyapparatus of an eleventh embodiment of the invention;

[0096]FIG. 16 is a circuit diagram of the switching power supplyapparatus of a twelfth embodiment of the invention;

[0097]FIG. 17 is a signal waveform diagram illustrating the start-upoperation of the switching power supply apparatuses shown in FIGS. 16and 19;

[0098]FIG. 18 is a circuit diagram of the switching power supplyapparatus of a thirteenth embodiment of the invention;

[0099]FIG. 19 is a circuit diagram of the switching power supplyapparatus of a fourteenth embodiment of the invention;

[0100]FIG. 20 is a circuit diagram of the switching power supplyapparatus of a fifteenth embodiment of the invention;

[0101]FIG. 21 is a circuit diagram of the switching power supplyapparatus of a sixteenth embodiment of the invention; and

[0102]FIG. 22 is a circuit diagram of the switching power supplyapparatus of a seventeenth embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0103] Hereinafter, embodiments of the present invention will bedescribed with reference to the drawings.

[0104] First Embodiment

[0105]FIG. 1 is a circuit diagram of the switching power supplyapparatus of a first embodiment of the invention.

[0106] In the switching power supply apparatus shown in FIG. 1, atransformer 3 has its primary coil 4 connected, at its one end, to apositive power supply line 1 and, at its other end, through a mainswitching device 5 to a negative power supply line 2. The main switchingdevice 5 is realized with, for example, an FET (field-effecttransistor). The transformer 3 has its secondary coil 6 connected, atits one end, through a diode 7 to an output line 25 and, at its otherend, to an output line 26. Between the output lines 25 and 26, there areconnected a capacitor 45 and an output voltage detector circuit 9. Theoutput terminal of the output voltage detector circuit 9 is connected byway of a line 9 a to the input terminal of a signal level checkercircuit 15 and to the input terminal of a switching controller circuit19 to feed each of them with a feedback signal.

[0107] An operating power source 16 has its negative end connected tothe negative power supply line 2, and has its positive end connected byway of a line 16 a to the power terminal of the signal level checkercircuit 15 and to the input terminal of a switch circuit 17. The signallevel checker circuit 15 feeds an operation control signal by way of aline 15 a to the control terminal of the switch circuit 17. The outputterminal of the switch circuit 17 is connected by way of a line 17 a tothe power terminal of the switching controller circuit 19. The outputterminal of the switching controller circuit 19 is connected to thecontrol terminal of the main switching device 5.

[0108] Next, the operation of the switching power supply apparatus ofthe first embodiment will be described. When a voltage from anonillustrated direct-current power source is applied between thepositive and negative power supply lines 1 and 2, the main switchingdevice 5, under the control of the switching controller circuit 19,performs switching operation, and thereby causes a high-frequencycurrent to flow through the primary coil 4 of the transformer 3. Thisinduces a high-frequency voltage in the secondary coil 6 of thetransformer 3. This high-frequency voltage is rectified by the diode 7and then smoothed by the capacitor 45, and is thereby converted into adirect-current voltage. This direct-current voltage is applied betweenthe output lines 25 and 26 so as to be output as the output voltage ofthe switching power supply apparatus.

[0109] The output voltage detector circuit 9 compares the output voltagebetween the output lines 25 and 26 with a predetermined referencevoltage, and feeds the result of the comparison in the form of afeedback signal by way of the line 9 a to the signal level checkercircuit 15 and the switching controller circuit 19. The switchingcontroller circuit 19 operates from the power supplied thereto from theoperating power source 16 through the switch circuit 17, and, bycontrolling the timing with which the main switching device 5 is turnedon and off according to the feedback signal, performs control in such away that a desired direct-current voltage is output between the outputlines 25 and 26.

[0110] When the load connected between the output lines 25 and 26 (theoutput terminals of the switching power supply apparatus) consumes asmall amount of electric power (i.e., in light-load operation), theoutput voltage between the output lines 25 and 26 (the output voltage ofthe switching power supply apparatus) tends to be higher. To correctthis, the output voltage detector circuit 9 outputs to the line 9 a afeedback signal having, for example, a higher level.

[0111] On the other hand, when the load connected between the outputlines 25 and 26 consumes a large amount of electric power (i.e., inheavy-load operation), the output voltage between the output lines 25and 26 tends to be lower. To correct this, the output voltage detectorcircuit 9 outputs to the line 9 a a feedback signal having, for example,a lower level.

[0112] When the output voltage between the output lines 25 and 26 (theoutput voltage of the switching power supply apparatus) is higher thanthe reference voltage, and the feedback signal output by way of the line9 a has a higher level, the signal level checker circuit 15 feeds anoperation control signal by way of the line 15 a to the switch circuit17 so as to turn the switch circuit 17 off.

[0113] With the switch circuit 17 turned off, the switching controllercircuit 19 ceases to be supplied with the voltage from the operatingpower source 16, and thus stops operating. As a result, the mainswitching device 5 stops operating, and thus permits the output voltagebetween the output lines 25 and 26 (the output voltage of the switchingpower supply apparatus) to decrease gradually.

[0114] As the output voltage decreases, the level of the feedback signalfrom the output voltage detector circuit 9 becomes, for example, lower.Then, the signal level checker circuit 15 feeds an operation controlsignal by way of the line 15 a to the switch circuit 17 so as to turnthe switch circuit 17 on. This causes the voltage from the operatingpower source 16 to be supplied to the switching controller circuit 19,and thus the switching controller circuit 19 restarts operating, andmakes the main switching device 5 perform switching operation.

[0115] Consequently, the output voltage between the output lines 25 and26 (the output voltage of the switching power supply apparatus)increases, and meanwhile the output voltage detector circuit 9 feeds ahigher-level feedback signal by way of the line 9 a to the signal levelchecker circuit 15. Then, the signal level checker circuit 15 turns theswitch circuit 17 off to stop the operation of the switching controllercircuit 19 and thereby stop the switching operation by the mainswitching device 5.

[0116] When the load connected between the output lines 25 and 26 is aheavy load that consumes a considerably large amount of electric power,the output voltage tends to be considerably low. In this case, thesignal level checker circuit 15 keeps the switch circuit 17 continuouslyon. Thus, the switching controller circuit 19 makes the main switchingdevice 5 perform switching operation continuously and thereby stabilizesthe output voltage.

[0117] As described above, bust switching operation is achieved byrepeatedly stopping switching operation when the output voltage of theswitching power supply apparatus increases and restarting switchingoperation when the output voltage decreases. This stabilizes the outputvoltage.

[0118] In burst switching operation, the operating power of the signallevel checker circuit 15 is supplied thereto without passing through theswitch circuit 17, and therefore the signal level checker circuit 15keeps operating even when switching operation is being stopped. However,the power consumption of the signal level checker circuit 15 is farlower than that of the switching controller circuit 19, and accordinglythe switching power supply apparatus operates with less powerconsumption, contributing to energy saving.

[0119] Second Embodiment

[0120]FIG. 2 is a circuit diagram of the switching power supplyapparatus of a second embodiment of the invention.

[0121] In the switching power supply apparatus shown in FIG. 2, atransformer 3 has its primary coil 4 connected, at its one end, to apositive power supply line 1 and, at its other end, through a mainswitching device 5 to a negative power supply line 2. The transformer 3has its secondary coil 6 connected, at its one end, through a diode 7 toan output line 25 and, at its other end, to an output line 26. Betweenthe output lines 25 and 26, there are connected a capacitor 45 and anoutput voltage detector circuit 9.

[0122] The output voltage detector circuit 9 is composed of two serialcircuits connected between the output lines 25 and 26, more specificallyone composed of a photocoupler 20, a resistor 21, and a shunt regulator22 and another composed of output voltage division resistors 23 and 24.The photocoupler 20 is composed of a photodiode 20 a and aphototransistor 20 b. The control terminal of the shunt regulator 22 isconnected to the node between the output voltage division resistors 23and 24. The shunt regulator 22 compares the voltage at the node betweenthe output voltage division resistors 23 and 24 with a reference voltagethat has previously been prepared internally, and permits a currentcommensurate with the result of the comparison to flow through thephotodiode 20 a.

[0123] An operating power source 16 has its negative end connected tothe negative power supply line 2, and has its positive end connected toa steady-state operating current supply line 16 a. A signal levelchecker circuit 15 is composed of a Zener diode 191, a resistor 201, acomparator 18, and a current detection resistor 28.

[0124] The Zener diode 191 has its cathode connected to the steady-stateoperating current supply line 16 a, and has its anode connected to oneend of the resistor 201 and to the non-inverting input terminal of thecomparator 18. The other end of the resistor 201 is connected to thenegative power supply line 2. The current detection resistor 28 has itsone end connected to the steady-state operating current supply line 16a, and has its other end connected to the inverting input terminal ofthe comparator 18 and to the collector of the phototransistor 20 b ofthe photocoupler 20.

[0125] The comparator 18 has its positive power terminal connected tothe steady-state operating current supply line 16 a, and has itsnegative power terminal connected to the negative power supply line 2.The output terminal of the comparator 18 is connected by way of a line15 a to the control terminal of a switch circuit 17. The emitter of thephototransistor 20 b is connected by way of a line 19 a to the controlterminal of the switching controller circuit 19. The output terminal ofthe switching controller circuit 19 is connected to the control terminalof the main switching device 5.

[0126] Next, the operation of the switching power supply apparatus ofthe second embodiment will be described. When a direct-current voltagefrom the operating power source 16 is applied between the positive andnegative power supply lines 1 and 2, the main switching device 5, underthe control of the switching controller circuit 19, performs switchingoperation, and thereby causes a high-frequency current to flow throughthe primary coil 4 of the transformer 3. This induces a high-frequencyvoltage in the secondary coil 6 of the transformer 3. Thishigh-frequency voltage is rectified by the diode 7 and then smoothed bythe capacitor 45, and is thereby converted into a direct-currentvoltage. This direct-current voltage is applied between the output lines25 and 26 so as to be output as the output voltage of the switchingpower supply apparatus.

[0127] The output voltage detector circuit 9 compares the output voltagebetween the output lines 25 and 26 with a predetermined referencevoltage, and feeds the result of the comparison in the form of afeedback signal, on one hand, via the node between the collector of thephototransistor 20 b and the current detection resistor 28 to the signallevel checker circuit 15 and, on the other hand, by way of the line 19 ato the switching controller circuit 19.

[0128] More specifically, in the output voltage detector circuit 9, theshunt regulator 22 compares the voltage at the node between the outputvoltage division resistors 23 and 24 with a reference voltage that haspreviously been prepared internally, and makes a current commensuratewith the result of the comparison flow through the photodiode 20 a. Thephototransistor 20 b supplies a current commensurate with the currentflowing through the photodiode 20 a from the operating power source 16through current detection resistor 28 to the switching controllercircuit 19. Thus, according to the current supplied, the switchingcontroller circuit 19 controls the switching operation of the mainswitching device 5, and thereby controls the output voltage of theswitching power supply apparatus (the voltage between the output lines25 and 26) so as to make it equal to a predetermined value.

[0129] In the signal level checker circuit 15, the comparator 18compares the voltage drop across the current detection resistor 28 withthe voltage of the current level check reference power generated by theZener diode 191 and the resistor 201, and feeds a signal commensuratewith the result of the comparison by way of the line 15 a to the switchcircuit 17. The Zener diode 191 may be replaced with a resistor.

[0130] When the load connected between the output lines 25 and 26 (theoutput terminals of the switching power supply apparatus) consumes asmall amount of electric power (i.e., in light-load operation), theoutput voltage between the output lines 25 and 26 (the output voltage ofthe switching power supply apparatus) tends to be higher. To correctthis, the output voltage detector circuit 9 increases the currentflowing through the phototransistor 20 b.

[0131] The comparator 18, as the result of comparing the current valuethrough the phototransistor 20 b with the reference current level set bythe current level check reference power, outputs a high-level operationcontrol signal, and feeds this high-level operation control signal byway of the line 15 a to the control terminal of the switch circuit 17,which is thereby turned off. This stops the supply of the supply voltageto the switching controller circuit 19, and thus the switchingcontroller circuit 19 stops operating. Consequently, the main switchingdevice 5 stops operating, and thus the output voltage of the switchingpower supply apparatus decreases gradually.

[0132] As the output voltage decreases, the current value through thephototransistor 20 b decreases. Then, the comparator 18, as the resultof comparing the current value through the phototransistor 20 b with thereference current level set by the current level check reference power,outputs a low-level operation control signal, and feeds this low-leveloperation control signal by way of the line 15 a to the control terminalof the switch circuit 17, which is thereby turned on. This starts thesupply of the supply voltage to the switching controller circuit 19, andthus the switching controller circuit 19 starts to operate.Consequently, the main switching device 5 starts to operate, and thusthe output voltage of the switching power supply apparatus increasesgradually.

[0133] As the output voltage increases, the current value through thephototransistor 20 b increases. Then, the comparator 18, as the resultof comparing the current value through the phototransistor 20 b with thereference current level set by the current level check reference power,outputs a high-level operation control signal, and feeds this high-leveloperation control signal by way of the line 15 a to the control terminalof the switch circuit 17, which is thereby turned off. This stops thesupply of the supply voltage to the switching controller circuit 19, andthus the switching controller circuit 19 stops operating. Consequently,the main switching device 5 stops operating, and thus the output voltageof the switching power supply apparatus decreases gradually. Thereafter,this sequence of control is repeated, and burst oscillation is therebymaintained. In this way, the output voltage of the switching powersupply apparatus is kept approximately constant.

[0134] Incidentally, among the operations described above, thosebelonging to the first part of the sequence described above, namely theturning off of the switch circuit 17, the stopping of the switchingoperation of the main switching device 5, the decrease in the outputvoltage, the decrease in the current through the phototransistor 20 b,and the output of the low-level signal from the comparator 18, are notperformed simultaneously, but, because of delays produced by variousportions of the circuit, performing all these operations requires acertain operation time, and, during this operation time, the switchingpower supply apparatus stops switching operation.

[0135] Likewise, the operations belonging to the second part of thesequence described above, namely the turning on of the switch circuit17, the starting of the switching operation of the main switching device5, the increase in the output voltage, the increase in the currentthrough the phototransistor 20 b, and the output of the high-levelsignal from the comparator 18, are not performed simultaneously, but,because of delays produced by various portions of the circuit,performing all these operations requires a certain operation time, and,during this operation time, the switching power supply apparatuscontinues switching operation.

[0136] The theory that the operation times described above help maintainthe periods during which the switching power supply apparatus keepsperforming and stops performing switching operation applies not only inthis embodiment but also in the first embodiment.

[0137] A small degree of hysteresis may be introduced in the control byslightly lowering the voltage level of the current level check referencepower fed to the non-inverting input terminal of the comparator 18 atthe same time that the switch circuit 17 is turned off and, likewise,slightly raising the voltage level of the current level check referencepower fed to the non-inverting input terminal of the comparator 18 atthe same time that the switch circuit 17 is turned on. This helps makelonger the periods during which the switching power supply apparatuskeeps performing and stops performing switching operation.

[0138] On the other hand, when the load connected between the outputlines 25 and 26 consumes a large amount of electric power (i.e., inheavy-load operation), the output voltage between the output lines 25and 26 tends to be lower. This causes the current flowing through thephototransistor 20 b to decrease, and thus causes the voltage dropacross the current detection resistor 28 to become lower than thevoltage across the Zener diode 191. Accordingly, the comparator 18outputs a low-level operation control signal, and thus the switchcircuit 17 is kept continuously on, permitting the switching powersupply apparatus to perform continuous switching operation.

[0139] Here, it should be noted that burst switching is achievedaccording to the result of comparison between the current value throughthe phototransistor 20 b and the reference current value (the voltageacross the Zener diode 191 as converted into a current value). Thesignal level of the feedback signal from the output voltage detectorcircuit 9 (i.e., the current value through the phototransistor 20 b)represents the load current value of the switching power supplyapparatus. Thus, it is possible to correctly set the load current valueat which switching between continuous switching operation and burstswitching operation is performed.

[0140] In burst switching operation, the output voltage fluctuates asdescribed earlier. However, since the signal level of the feedbacksignal, i.e., the current value through the phototransistor 20 b, alsorepresents the output voltage value of the switching power supplyapparatus as has already been described and will also be describedlater, it is possible to correctly set the upper and lower limits of theoutput voltage.

[0141] The signal level of the feedback signal may be detected on theline 19 a leading to the control terminal of the switching controllercircuit 19. However, as will be described later, this configurationcannot cope with a case where a current flows out of the switchingcontroller circuit 19 via its control terminal. That is, as the supplyof operating power to the switching controller circuit 19 is turned onand off, the current value flowing out of it via its control terminalvaries, and thus the voltage value at the control terminal no longercorrectly represents the output voltage and the load current asdescribed earlier.

[0142] In this way, burst switching operation is achieved by repeatedlystopping switching operation when the output voltage of the switchingpower supply apparatus increases and restarting stitching operation whenthe output voltage decreases. This helps stabilize the output voltage.

[0143] In burst switching operation, the operating power of the signallevel checker circuit 15 is supplied thereto without passing through theswitch circuit 17, and therefore the signal level checker circuit 15keeps operating even when switching operation is being stopped. However,the power consumption of the signal level checker circuit 15 is farlower than that of the switching controller circuit 19, and accordinglythe switching power supply apparatus operates with less powerconsumption, contributing to energy saving.

[0144] Third Embodiment

[0145]FIG. 3 is a circuit diagram of the switching power supplyapparatus of a third embodiment of the invention. FIG. 3 is a circuitdiagram showing the detailed circuit configuration of the operatingpower source 16 shown in FIGS. 1 and 2. In FIG. 3, such circuitcomponents that find their counterparts in FIGS. 1 and 2 are identifiedwith the same reference numerals, and their explanations will not berepeated.

[0146] In FIG. 3, the operating power of the switching controllercircuit 19 is supplied thereto by way of a start-up current supply line29 a by way of which a start-up current is supplied from the positivepower supply line 1 through a start-up resistor 29, or by way of asteady-state operating currant supply line 16 a by way of which avoltage induced in a subsidiary coil 32 of the transformer 3 is suppliedthrough a serial circuit composed of a plurality of diodes 30 and 31.The operating power of the signal level checker circuit 15 and thephototransistor 20 b of the photocoupler 20 is supplied thereto fromsubsidiary control power extracted from the node between the diodes 30and 31.

[0147] The circuit corresponding to the operating power source 16described earlier is composed of the subsidiary coil 32 of thetransformer 3, the diode 31, a capacitor 33, the diode 30, the start-upresistor 29, and a capacitor 46. In this switching power supplyapparatus, at the start of start-up, when a direct-current voltage froma nonillustrated direct-current power source is applied between thepositive and negative power supply lines 1 and 2, a charge current flowsthrough the capacitor 46 by way of the start-up resistor 29, and, aswill be described later, since the switch circuit 17 is on, when thecharge voltage of the capacitor 46 reaches a predetermined voltagelevel, the switching controller circuit 19 starts to operate and startsto supply the main switching device 5 with a drive signal.

[0148] Thus, the switching power supply apparatus starts switchingoperation, and a high-frequency voltage is induced in the subsidiarycoil 32 of the transformer 3. This high-frequency voltage is rectifiedand smoothed by the diode 31 and the capacitor 33, and is therebyconverted into a direct-current voltage. The phototransistor 20 b andthe comparator 18 operate from the operating power supplied thereto fromthe capacitor 33, and operate in such a way as to keep the outputvoltage of the switching power supply apparatus at a predetermined valueand achieve burst switching control when the load is light as describedearlier.

[0149] During the start-up operation of the switching power supplyapparatus, the diode 30 prevents a current from flowing from thepositive power supply line 1 through the start-up resistor 29 to thecapacitor 33, and thus helps shorten the time required for the chargevoltage of the capacitor 46 to reach the predetermined voltage level. Oncompletion of the start-up of the switching power supply apparatus, thecapacitor 46 is charged mainly by the current fed thereto from thecapacitor 33 through the diode 30, and supplies operating power to theswitching controller circuit 19 through the switch circuit 17.

[0150] When the switching power supply apparatus starts to start up, thecharge voltage of the capacitor 33 is zero, and therefore the comparator18 is not operating. However, since the output terminal of thiscomparator 18 is pulled down by a resistor 62, the switch circuit 17 isin an on state.

[0151] Likewise, when the switching power supply apparatus starts tostart up, the charge voltage of the capacitor 33 is zero, and thereforeno current flows through the phototransistor 20 b. Thus, the switchingcontroller circuit 19 controls the main switching device 5 on theassumption that the output voltage of the switching power supplyapparatus is lower than the predetermined value. Thereafter, as theoutput voltage of the switching power supply apparatus increases, thecharge voltage of the capacitor 33 increases until a current flowsthrough the phototransistor 20 b, when the switching power supplyapparatus starts to operate in a predetermined steady state.

[0152] As described above, during the period after the switching powersupply apparatus starts to start up until it starts to operate in thesteady state, the switching controller circuit 19 and the switch circuit17 operates by using as operating power the charge voltage of thecapacitor 46. Accordingly, to prevent the charge voltage of thecapacitor 46 from becoming lower than the permitted minimum operatingvoltage during that period, the capacitor 46 needs to be given asufficiently high capacitance.

[0153] By increasing the resistance of the start-up resistor 29, it ispossible to reduce the power loss through the start-up resistor 29.However, making the resistance too high results in lengthening the timerequired to charge the capacitor 46 when the switching power supplyapparatus starts up, slowing down its start-up.

[0154] In this embodiment, when the switching power supply apparatusstarts up, the diode 30 prevents the charge accumulated in the capacitor46 from flowing out of it to the phototransistor 20 b and the comparator18. This helps reduce the time required for start-up. Moreover, byincreasing the resistance of the start-up resistor 29, it is possible toreduce power consumption.

[0155] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the switch circuit 17 provided in the lineby way of which the switching controller circuit 19 is supplied withoperating power. Moreover, in burst switching control, while theswitching operation of the main switching device 5 is being stopped, thesupply of operating power to the switching controller circuit 19 is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0156] Fourth Embodiment

[0157]FIG. 4 is a circuit diagram of the switching power supplyapparatus of a fourth embodiment of the invention. In FIG. 4, suchcircuit components that find their counterparts in FIGS. 1 to 3 areidentified with the same reference numerals, and their explanations willnot be repeated. The switching power supply apparatus of this embodimentincorporates a PWM (pulse-width modulation) control IC, for example onewith the product number FA5511 manufactured by Fuji Electric Co., Ltd.In FIG. 4, FA5511 is shown as an IC 38.

[0158]FIG. 6 shows an outline of the configuration of FA5511. In FIG. 6,when operating power is supplied to a Vcc terminal T6, this operatingpower is supplied to an output buffer 101, an operation control circuit102, and a 5 V voltage regulator 103. When the voltage supplied via theVcc terminal T6 becomes higher than a predetermined operation startingvoltage, the 5 V voltage regulator 103 is brought into an output-enabledstate, and thus supplies stabilized 5 V power, on one hand, by way of aninternal supply line 104 to a PWM logic circuit 105 and an OSC(oscillation circuit ) 106 and, on the other hand, by way of theinternal supply line 104 and then through a diode 107 and a resistor 108to an FB terminal T2.

[0159] An internal power terminal T7 is connected to the internal supplyline 104, and to this internal power terminal T7 is externally connecteda capacitor 40 for eliminating noise from the internal supply line 104.This capacitor 40 prevents noise from being superimposed on the powersupplied by way of the internal supply line 104, and thereby preventserroneous control.

[0160] The oscillation frequency of the OSC 106 is set by the resistanceof a resistor 36 that is externally connected via a terminal T1. Theoscillation signal generated by the OSC 106 is fed to the PWM logiccircuit 105. The FB terminal T2 is pulled up to the internal supply line104 through a serial circuit composed of a diode 107 and a resistor 108,and thus a voltage divided by the serial circuit and a circuit elementexternally connected to the FB terminal T2 is supplied to the PWM logiccircuit 105.

[0161] The PWM logic circuit 105 performs, in the manner that will bedescribed later, logic calculation on the voltage level at the FBterminal T2, the voltage level at a CS terminal T8, which will bedescribed later, and the oscillation signal fed from the OSC 106, andfeeds the output buffer 101 with a drive signal for driving the mainswitching device 5 (see FIG. 4). The output buffer 101 current-amplifiesthe drive signal, and then feeds it as the drive signal to the mainswitching device 5, which is externally connected via an output terminalT5.

[0162] Via a terminal T3, a current detection signal from the mainswitching device 5 is fed in. When the current flowing through the mainswitching device 5 exceeds a predetermined level, the PWM logic circuit105 shuts off the drive signal for the main switching device 5 (reducesit to a low level) to protect the main switching device 5. A terminal T4serves as a common ground terminal of the internal circuit of FA5511,and is connected to the negative power supply line 2 (see FIG. 4) of theswitching power supply apparatus.

[0163] To a CS terminal T8 is externally connected a capacitor 41. Atthe same time that the operation control circuit 102 outputs an outputenable signal to the 5 V voltage regulator 103 as described earlier, theoperation control circuit 102 feeds the capacitor 41 with a weakcurrent, with which the capacitor 41 is charged gradually. When theswitching power supply apparatus is operating in the steady state, theoperation control circuit 102 controls the charge voltage of thecapacitor 41 in such a way that it does not exceed a predeterminedvoltage level.

[0164] When the potential at the CS terminal T8 is forcibly turned lowwith an external circuit, the operation control circuit 102 disables the5 V voltage regulator 103 and thereby stops the supply of power to theinternal supply line 104, and simultaneously the 5 V voltage regulator103 outputs a disable signal to the output buffer 101. Thus, when thepotential at the CS terminal T8 is forcibly turned low with an externalcircuit, the power consumption by FA5511 is greatly reduced.

[0165] The switching power supply apparatus of this embodiment exploitsthe above-described function of FA5311. Specifically, when the outputvoltage of the switching power supply apparatus is high, and thus thesignal level of the feedback signal is high, the signal level checkercircuit 15 (see FIG. 4) forcibly turns the potential at the CS terminalT8 low with an external circuit, and thereby stops the operation of theoutput buffer 101, PWM logic circuit 105, and OSC 106. This causes theswitching power supply apparatus to stop operating, and as a result thesignal level of the feedback signal decreases. Then, the signal levelchecker circuit 15 ceases to forcibly turn the potential at the CSterminal T8 low, and thereby restarts the switching power supplyapparatus. In this way, in light-load operation of the switching powersupply apparatus, burst switching operation is achieved.

[0166]FIG. 5 shows, for reference proposes, the circuit configuration ofa switching power supply apparatus having a typical circuitconfiguration in a case where it adopts FA5511. In FIG. 5, such circuitcomponents that find their counterparts in FIG. 4 are identified withthe same reference numerals, and their explanations will not berepeated. FIG. 7 shows the waveforms of the signals observed at relevantpoints in the switching power supply apparatus during the period afterit starts to start up until it starts to operate in the steady state. InFIG. 7, at (a) is shown the voltage 701 across the capacitor 46 shown inFIG. 5; at (b) are shown the voltage 702 at the FB terminal T2 of the IC38, i.e., FA5511, shown in FIG. 5, the oscillation signal 703 that theOSC 106 (see FIG. 6) feeds to the PWM logic circuit 105 (see FIG. 6),and the voltage 704 at the CS terminal T8; at (c) is shown the outputsignal 705 output via the output terminal T5.

[0167] Now, with reference to FIGS. 5 and 7, the operation of thisswitching power supply apparatus will be described. First, when, at atime point t0, a direct-current voltage is applied between the positiveand negative power supply lines 1 and 2, the voltage 701 across thecapacitor 46 increases gradually owing to a charge current suppliedthereto through the start-up resistor 29. When, at a time point t1, thevoltage reaches the predetermined operation starting voltage of FA5511,the voltage on the internal supply line 104 inside the IC 38 rises asdescribed earlier, and thus the OSC 106, PWM logic circuit 105, andoutput buffer 101 start to operate.

[0168] Thus, the OSC 106 feeds the PWM logic circuit 105 with anoscillation signal 703 having constant upper and lower limits and aconstant period, and, as a result of the capacitor 41 being charged withthe weak current fed thereto from the operation control circuit 102, thevoltage 704 at the CS terminal T8 increases gradually. At the time pointt1, the voltage between the output lines 25 and 26 is still zero, andtherefore no current flows through the shunt regulator 22 and thephototransistor 20 b. Thus, the voltage 702 at the FB terminal T2 of theIC 38 is high.

[0169] When whichever of the voltage 704 at the CS terminal T8 and thevoltage 702 at the FB terminal T2 is lower is higher than the voltage ofthe oscillation signal 703 output from the OSC 106, the PWM logiccircuit 105 outputs an output signal (a pulse signal) 705 of which thelevel is higher than the voltage at the output terminal T5 of the outputbuffer 101. Thus, during the period from the time point t1 to a timepoint t2, during which the level of the voltage 704 at the CS terminalT8 is lower than the level of the oscillation signal 703 output from theOSC 106, the output signal 705 remains low. At the time point t2, whenthe level of the voltage 704 at the CS terminal T8 momentarily exceedsthe level of the oscillation signal 703 of the OSC 106, the outputsignal 705 becomes high and then remains high for the correspondingperiod, turning the main switching device 5 on.

[0170] Thereafter, as the voltage 704 increases, the period during whichthe output signal 705 remains high becomes increasingly long, andcorrespondingly the power supplied from the secondary coil 6 of thetransformer 3 through the diode 7 to between the output lines 25 and 26increases. Thus, the voltage between the output lines 25 and 26increases until, at a time point t3, a current starts to flow throughthe shunt regulator 22 and the phototransistor 20 b, when the voltage702 at the FB terminal T2 starts to decrease.

[0171] Next, when, at a time point t5, the voltage 702 at the FBterminal T2 becomes lower than the voltage 704 at the CS terminal T8,the period during which the output signal 705 at the output terminal T5is high is determined by the result of comparison between the level ofthe oscillation signal 703 of the OSC 106 and the voltage 702 at the FBterminal T2. Since the level of the voltage 702 represents the feedbacksignal output from the output voltage detector circuit 9, the switchingpower supply apparatus now starts to operate in the steady state inwhich it outputs a predetermined voltage.

[0172] On the other hand, the charge voltage 701 of the capacitor 46tends to slightly decrease during the period from the time point t1 tothe time point t3, because during that period more current flows to theVcc terminal T6 than is supplied from the start-up resistor 29. However,this decrease is so controlled as not to go below the minimum operatingVcc voltage of the IC 38, i.e., FA5511, by giving the capacitor 46 asufficiently high capacitance.

[0173] As described earlier, the output voltage of the switching powersupply apparatus increases, and correspondingly the charge voltage 701of the capacitor 46 starts to increase at a time point t4 and reachesthe steady-state stable voltage at a time point t6.

[0174] It is to be understood that the circuit configuration of theswitching power supply apparatus explained with reference to FIG. 5 is amere example of a typical circuit configuration in a case where FA5511is adopted, and thus does not incorporate the function of achievingburst switching in light-load operation which will be described later inconnection with this particular embodiment.

[0175] Next, the operation of the switching power supply apparatus ofthis embodiment shown in FIG. 4 will be described with reference to asignal waveform diagram shown in FIG. 8. In FIG. 8, at (a) is shown thevoltage 801 across the capacitor 46 shown in FIG. 4; at (b) are shownthe voltage 804 at the FB terminal T2 of the IC 38, i.e., FA5511, shownin FIG. 4, the oscillation signal 803 that the OSC 106 (see FIG. 6)feeds to the PWM logic circuit 105 (see FIG. 6), and the voltage 805 atthe CS terminal T8 of the IC 38; at (c) is shown the output signal 806output via the output terminal T5 of the IC 38.

[0176] First, when, at a time point T0, a direct-current voltage isapplied between the positive and negative power supply lines 1 and 2,the voltage 801 across the capacitor 46 increases gradually owing to acharge current supplied thereto through the start-up resistor 29. When,at a time point T1, the voltage reaches the predetermined operationstarting voltage of FA5511, the voltage on the internal supply line 104inside the IC 38 rises as described earlier, and thus the OSC 106, PWMlogic circuit 105, and the output buffer 101 start to operate.

[0177] Thus, the OSC 106 feeds the PWM logic circuit 105 with anoscillation signal 803 having constant upper and lower limits and aconstant period, and, as a result of the capacitor 41 being charged withthe weak current fed thereto from the operation control circuit 102, thevoltage 805 at the CS terminal T8 increases gradually. At the time pointT1, the charge voltage of the capacitor 33 is zero, the output currentof the signal level checker circuit 15 is zero, and the switch of astart-up corrector circuit 35 is, as will be described later, off.Accordingly, the voltage 804 at the FB terminal T2 of the IC 38 is adivision voltage that results from voltage division by the diode 107provided inside the IC 38, the resistor 108, and a resistor 39 a (seeFIG. 4).

[0178] This division voltage has its value set to be slightly higherthan the lower-limit voltage level of the oscillation signal 803.

[0179] When whichever of the voltage 805 at the CS terminal T8 and thevoltage 804 at the FB terminal T2 is lower is higher than the voltagelevel of the oscillation signal 803 output from the OSC 106, the PWMlogic circuit 105 outputs an output signal 806 via the output terminalT5 of the output buffer 101.

[0180] Thus, during the period from the time point T1 to a time pointT2, during which the level of the voltage 805 at the CS terminal T8 islower than the level of the oscillation signal 703 output from the OSC106, the output signal 806 remains low. At the time point T2, when thelevel of the voltage 805 at the CS terminal T8 momentarily exceeds thelevel of the oscillation signal 803 of the OSC 106, the output signal705 becomes high and then remains high for the corresponding period,turning the main switching device 5 on.

[0181] This causes the voltage between the output lines 25 and 26 toslightly increase, and thus causes the charge voltage of the capacitor33 to increase in such a way as to correspond to the increase in thevoltage between the output lines 25 and 26. Consequently, a currentstarts to be supplied from the capacitor 33 through the signal levelchecker circuit 15 to the FB terminal T2 of the IC 38, and thus thevoltage 804 at the FB terminal T2 starts to increase.

[0182] When the current value through the phototransistor 20 b is lowerthan a predetermined value set within the signal level checker circuit15, the signal level checker circuit 15 supplies a current to the FBterminal T2 of the IC 38; by contrast, when the current value throughthe phototransistor 20 b is higher than the predetermined value setwithin the signal level checker circuit 15, the signal level checkercircuit 15 feeds a current to a CS terminal controller circuit 37, butsupplies no current to either of the FB terminal T2 and the CS terminalT8.

[0183] While the signal level checker circuit 15 is supplying a currentto the FB terminal T2 of the IC 38, when the current value through thephototransistor 20 b increases, the signal level checker circuit 15decreases the supply current (inverted feedback signal); by contrast,when the current value through the phototransistor 20 b decreases, thesignal level checker circuit 15 increases the supply current (invertedfeedback signal).

[0184] The supply current also depends on the operating power of thesignal level checker circuit 15; that is, it also depends on the chargevoltage of the capacitor 33. Thus, as described earlier, after theswitching power supply apparatus starts to start up, as the voltagebetween the output lines 25 and 26 increases, and thus as the chargevoltage of the capacitor 33 increases, the supply current increases.

[0185] Thereafter, when the supply current increases until thesteady-operation state is reached in which the voltage between theoutput lines 25 and 26 are stabilized, the voltage between the outputlines 25 and 26 and the charge voltage of the capacitor 33 arestabilized at constant values determined by the predetermined outputvoltage of the switching power supply apparatus and the winding ratiobetween the secondary coil 6 and the subsidiary coil 32 of thetransformer 3. Thus, now, the supply current depends solely on thecurrent value through the phototransistor 20 b as described above.

[0186] Next, after the time point T2, as the voltage 805 at the CSterminal T8 increases, the period during which the output signal 806output via the output terminal T5 remains high becomes increasinglylong, thus the voltage between the output lines 25 and 26 increases,thus the charge voltage of the capacitor 33 increases, and thus thecurrent supplied from the signal level checker circuit 15 increases. Asa result of this course of events, the voltage 804 at the FB terminal T2increases gradually.

[0187] After a time point T3, when the voltage 805 at the CS terminal T8becomes higher than the voltage 804 at the FB terminal T2, as describedearlier, the PWM logic circuit 105 compares the voltage 804 at the FBterminal T2 with the oscillation signal 803 of the OSC 106, and,according to the result of the comparison, outputs the output signal 806through the output buffer 101 via the output terminal T5 so as to feedthe output signal 806 as the drive signal to the main switching device5.

[0188] As described above, the charge voltage of the capacitor 33depends on the voltage between the output lines 25 and 26 and thewinding ratio between the secondary coil 6 and subsidiary coil 32 of thetransformer 3. Thus, after the time point T2, as the voltage between theoutput lines 25 and 26 increases, the charge voltage of the capacitor 33increases describing a curve 802 shown at (a) in FIG. 8. When, at a timepoint T4, the voltage of the capacitor 33 becomes higher than apredetermined value set within the start-up corrector circuit 35, thestart-up corrector circuit 35 turns on a switch provided therein so asto connect a resistor 39 b in parallel with the resistor 39 a.

[0189] Consequently, the voltage 804 at the FB terminal T2 momentarilydecreases, but, since the voltage level after this decrease is higherthan the lower limit of the oscillation signal 803, although thehigh-level period of the output signal 806 at the output terminal T5 ismomentarily shortened, the main switching device 5 continues switchingoperation. Thus, the voltage between the output lines 25 and 26 and thecharge voltage of the capacitor 33 still continues to increase, and thevoltage 804 at the FB terminal T2 starts to increase again.

[0190] Immediately before a time point T6, when the voltage resultingfrom voltage division by the resistors 23 and 24 reaches the comparisonreference value provided within the shunt regulator 22, a current startsto flow through the shunt regulator 22 , photodiode 20 a, andphototransistor 20 b. Thus, the supply current from the signal levelchecker circuit 15 stops increasing, the voltage 804 at the FB terminalT2 stops increasing, and the switching power supply apparatus starts tooperate in the steady state.

[0191] In this steady-state operation, for example, when the voltagebetween the output lines 25 and 26 increases, the voltage resulting fromvoltage division by the resistors 23 and 24 increases, thus the currentthrough the shunt regulator 22, photodiode 20 a, and phototransistor 20b increases, thus the supply current from the signal level checkercircuit 15 decreases, thus the voltage 804 at the FB terminal T2decreases, then the PWM logic circuit 105 compares the oscillationsignal 803 of the OSC 106 with the voltage 804 at the FB terminal T2 andas a result outputs via the output terminal T5 of the IC 38 an outputsignal (drive signal) 806 of which the high-level period is short, thusthe on-state duty of the main switching device 5 becomes shorter, andthus the current supplied through the diode 7 to the output line 25decreases. As a result of this course of events, the voltage between theoutput lines 25 and 26 is decreased.

[0192] By contrast, when the voltage between the output lines 25 and 26decreases, the voltage resulting from voltage division by the resistors23 and 24 decreases, thus the current through the shunt regulator 22,photodiode 20 a, and phototransistor 20 b decreases, thus the supplycurrent from the signal level checker circuit 15 increases, thus thevoltage 804 at the FB terminal T2 increases, then the PWM logic circuit105 compares the oscillation signal 803 of the OSC 106 with the voltage804 at the FB terminal T2 and as a result outputs via the outputterminal T5 of the IC 38 an output signal (drive signal) 806 of whichthe high-level period is long, thus the on-state duty of the mainswitching device 5 becomes longer, and thus the current supplied throughthe diode 7 to the output line 25 increases. As a result of this courseof events, the voltage between the output lines 25 and 26 is increased.

[0193] Through this sequence of operations, the voltage between theoutput lines 25 and 26 is stabilized at a predetermined value.Consequently, the charge voltage of the capacitor 33 is also stabilized,and thus the amount of current supplied from the signal level checkercircuit 15 depends solely on the current through the phototransistor 20b.

[0194] The start-up corrector circuit 35 switches the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2between when the switching power supply apparatus is starting up andwhen it is operating in the steady state. This ensures that theswitching power supply apparatus reliably performs switching operation.

[0195] Specifically, when the switching power supply apparatus starts tostart up, the charge voltage of the capacitor 33 is zero, and thecurrent supplied from the signal level checker circuit 15 is zero. Thus,the resistor 39 a is given a high resistance so that, as describedearlier, the voltage resulting from voltage division by the diode 107provided within the IC 38, the resistor 108, and the resistor 39 a (seeFIG. 4) is higher than the lower limit of the oscillation signal of theOSC 106.

[0196] If this is not the case, even after the voltage level at the CSterminal T8 of the IC 38 increases, the voltage level at the FB terminalT2 remains lower than the lower limit of the oscillation signal of theOSC 106, and thus the PWM logic circuit 105 does not output a high-levelsignal via the output terminal T5. This makes it impossible for theoutput voltage of the switching power supply apparatus to rise.

[0197] On the other hand, in steady-state operation, if the resistancebetween the FB terminal T2 of the IC 38 and the negative power supplyline 2 is kept high, for example, when the output voltage of theswitching power supply apparatus increases as a result of the switchingpower supply apparatus operating in a no-load state, as the outputvoltage is stabilized, even when the signal level checker circuit 15stops the supply current, the voltage resulting from voltage division bythe diode 107 within the IC 38, the resistor 108, and the resistor 39 a(see FIG. 4) does not fall below the lower limit of the oscillationsignal of the OSC 106, and thus, quite inconveniently, the voltage atthe FB terminal T2 cannot be so controlled as to decrease the outputvoltage.

[0198] To overcome this inconvenience, when the switching power supplyapparatus is starting up, as the charge voltage of the capacitor 33increases, immediately before a current starts to flow through thephototransistor 20 b, the signal level checker circuit 15 additionallyconnects the resistor 39 b so as to reduce the resistance between the FBterminal T2 of the IC 38 and the negative power supply line 2.

[0199] As described above, the switching power supply apparatus performsburst switching operation in light-load operation. This helps reducepower loss in light-load operation.

[0200] As described earlier, in the switching power supply apparatus,the output voltage tends to increase in light-load operation. To correctthis, the current value through the phototransistor 20 b is increased.This current through the phototransistor 20 b is made to flow throughthe current-detection resistor 34, and the voltage across thiscurrent-detection resistor 34 is compared with the reference voltageprovided within the signal level checker circuit 15 so that, when thevoltage across the current-detection resistor 34 is higher than thereference voltage, the signal level checker circuit 15 feeds the supplycurrent to the CS terminal controller circuit 37 and stops the supply ofcurrent to the FB terminal T2 of the IC 38.

[0201] On detecting the supply current, the CS terminal controllercircuit 37 turns on the switch provided therein to turn the voltage atthe CS terminal T8 of the IC 38 low. When the voltage at the CS terminalT8 is turned low, the operation control circuit 102 turns off the outputof the 5 V voltage regulator 103, and stops the supply of pull-upcurrent to the FB terminal T2 and the supply of operating power to theOSC 106 and the PWM logic circuit 105.

[0202] Moreover, the operation control circuit 102 feeds a disablesignal to the output buffer 101 to stop the operation of the outputbuffer 101. This stops the feeding of the drive signal from the outputterminal T5 of the IC 38 to the main switching device 5, and thus theswitching power supply apparatus stops switching operation.

[0203] Consequently, as the voltage between the output lines 25 and 26decreases, the voltage resulting from voltage division by the resistors23 and 24 decreases, thus the current flowing through the shuntregulator 22, photodiode 20 a, and phototransistor 20 b decreases, thusthe voltage across the current-detection resistor 34 decreases, and thenthe signal level checker circuit 15 compares the voltage across thecurrent-detection resistor 34 with the reference voltage providedtherein and judges the voltage across the current-detection resistor 34to be lower. Thus, the signal level checker circuit 15 feeds the supplycurrent to the FB terminal T2 of the IC 38 and stops the supply ofcurrent to the CS terminal controller circuit 37.

[0204] Consequently, the CS terminal controller circuit 37 turns off theswitch provided therein to turn the voltage at the CS terminal T8 of theIC 38 high. Thus, the operation control circuit 102 turns the 5 Vvoltage regulator 103 on, and restarts the supply of pull-up current tothe FB terminal T2 and the supply of operating power to the OSC 106 andthe PWM logic circuit 105. Moreover, the operation control circuit 102feeds an enable signal to the output buffer 101 to restart the operationof the output buffer 101.

[0205] This restarts the supply of the drive signal from the outputterminal T5 of the IC 38 to the main switching device 5, and thus theswitching power supply apparatus restarts switching operation.

[0206] Thereafter, when the voltage between the output lines 25 and 26increases again, switching operation is stopped as described above.When, consequently, the voltage between the output lines 25 and 26increases decreases, and the voltage across the current-detectionresistor 34 decreases, switching operation is restarted as describedabove. Through repetition of these operations, burst switching operationis achieved.

[0207] In this burst switching state, as the output current of theswitching power supply apparatus is increased, the time comes when,during the period of switching operation, the voltage level across thecurrent-detection resistor 34 no longer reaches the level of thereference voltage provided within the signal level checker circuit 15.This is the start of a continuous switching mode.

[0208] Adopting the technique of the fourth embodiment described abovemakes it possible to carry out the present invention on a practicalbasis simply by adding an additional circuit to a commercially availablePWM control IC, for example one with the product number FA5511manufactured by Fuji Electric Co., Ltd. or an equivalent.

[0209] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal circuit portions of the IC 38 is also stopped. This helpsreduce the power loss suffered while the switching operation is beingstopped, and thus helps reduce the power consumption of the apparatus asa whole.

[0210] Here, the principal circuit portions of the IC 38 are the OSC106, PWM logic circuit 105, FB terminal T2, and output buffer 101.

[0211] Moreover, the start-up corrector circuit 35 so operates that,when the switching power supply apparatus shifts from start-up operationto steady-state operation, the resistor 39 b is connected in parallelwith the resistor 39 a to reduce the resistance between the FB terminalT2 of the IC 38 and the negative power supply line 2. This lowers thepotential at the FB terminal T2, and thereby ensures that the switchingpower supply apparatus performs reliable output voltage stabilizingcontrol when operating in the steady state.

[0212] Fifth Embodiment

[0213]FIG. 9 is a circuit diagram of the switching power supplyapparatus of a fifth embodiment of the invention. In FIG. 9, suchcircuit components that find their counterparts in FIG. 4 are identifiedwith the same reference numerals, and their explanations will not berepeated. In the switching power supply apparatus of the fifthembodiment, the configuration of the signal level checker circuit 15,start-up corrector circuit 35, and CS terminal controller circuit 37 isshown in detail.

[0214] The signal level checker circuit 15 is composed of PNP-typetransistors 47 and 48 and resistors 49, 50, and 51. The start-upcorrector circuit 35 is composed of a Zener diode 54, resistors 55, 56,and 39 b, and an NPN-type transistor 57. The CS terminal controllercircuit 37 is composed of an NPN-type transistor 53 and a resistor 52.In the following descriptions, both PNP-type and NPN-type transistorsare referred to simply as transistors.

[0215] In the signal level checker circuit 15, the emitter of thetransistor 47 and the emitter of the transistor 48 are connectedtogether, and between these emitters and the capacitor 33 is connectedthe resistor 49. The base of the transistor 47 is connected to the nodebetween the emitter of the phototransistor 20 b and thecurrent-detection resistor 34.

[0216] The base of the transistor 48 is connected to the point (the nodebetween the resistors 50 and 51) at which a reference voltage isgenerated by dividing the voltage across the capacitor 33 with theserially connected resistors 50 and 51. The collector of the transistor47 is connected to the FB terminal T2 of the IC 38, i.e., FA5511, andthe collector of the transistor 48 is connected to the base of thetransistor 53 provided in the CS terminal controller circuit 37.

[0217] Configured as described above, the signal level checker circuit15 operates in the following manner when the switching power supplyapparatus is operating in the steady state.

[0218] As described earlier, the charge voltage of the capacitor 33 isstabilized, and a voltage produced by dividing the charge voltage withthe resistors 50 and 51 is used as a reference voltage. Let thisreference voltage be Eb. At the node between the phototransistor 20 band the current-detection resistor 34, there appears a voltage roughlyproportional to the signal level of the feedback signal output from theoutput voltage detector circuit 9. When this voltage is lower than thereference voltage Eb, the transistor 47 is on and the transistor 48 isoff. Thus, a current Ia given by formula (1) below flows through thecollector of the transistor 47.

Ia=(Ea−Ee−Va)/Rd   (1)

[0219] In formula (1) above, Ea represents the charge voltage of thecapacitor 33, Ee represents the voltage at the node between thephototransistor 20 b and the current-detection resistor 34 (i.e., thebase voltage of the transistor 47), Va represents the forward voltagebetween the base and emitter of the transistor 47, and Rd represents theresistance of the resistor 49.

[0220] Accordingly, as the current flowing through the phototransistor20 b increases, the current supplied to the FB terminal T2 of the IC 38is reduced, and, as the current flowing through the phototransistor 20 bdecreases, the current supplied to the FB terminal T2 is increased.Moreover, when the current flowing through the phototransistor 20 bfurther increases, the voltage at the node between the phototransistor20 b and the current-detection resistor 34 becomes higher than thereference voltage Eb. This turns the transistor 47 off and thetransistor 48 on, and thus a current is supplied from the collector ofthe transistor 48 to the CS terminal controller circuit 37.

[0221] Next, the CS terminal controller circuit 37 will be described.The transistor 53 has its collector connected to the CS terminal T8 ofthe IC 38, i.e., FA5511, has its emitter connected to the negative powersupply line 2, and has its base connected to the output end of thesignal level checker circuit 15.

[0222] Accordingly, when a current is supplied from the signal levelchecker circuit 15, the transistor 53 turns on, and thereby turns thevoltage at the CS terminal T8 of the IC 38 low.

[0223] A diode 58 is connected between the CS terminal T8 of the IC 38and the capacitor 41, and this diode 58 serves to quicken thefluctuation of the voltage level at the CS terminal T8 of the IC 38 andthereby quicken the speed of switching between an oscillating state anda resting state in burst oscillation operation.

[0224] If this diode 58 is not inserted, i.e., if the CS terminal T8 ofthe IC 38 is connected directly to the capacitor 41, when the transistor53 turns on, the voltage at the CS terminal T8 does not turn low untilthe charge accumulated in the capacitor 41 is depleted. This delays thestopping of switching operation. On the other hand, when the transistor53 turns off, it takes time for the capacitor 41 to be charged by thecurrent supplied from the operation control circuit 102 to above thelower-limit voltage level of the oscillation signal of the OSC 106. Thisdelays the restarting of switching operation. As a result, in burstswitching operation, it occurs that, when the load current of theswitching power supply apparatus abruptly increases during the period inwhich switching operation is not being performed, the output voltagedecreases by an increased amount.

[0225] In applications where the effects of the delays in the stoppingand restarting of switching operation can be ignored, it is notnecessary to insert the diode 58.

[0226] Next, the start-up corrector circuit 35 will be described. In thestart-up corrector circuit 35, the Zener voltage of the Zener diode 54is the aforementioned predetermined voltage that is so set that, whenthe charge voltage of the capacitor 33 increases above it, thetransistor 57 is turned on. Accordingly, when the charge voltage of thecapacitor 33 increases above the Zener voltage (predetermined voltage),the transistor 57 is supplied with its base current from the capacitor33 through the Zener diode 54 and the resistor 55. This turns thetransistor 57 on, which thus connects the resistor 39 b in parallel withthe resistor 39 a and thereby lowers the voltage at the FB terminal T2of the IC 38.

[0227] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal circuit portions of the IC 38, namely the OSC 106, PWMlogic circuit 105, FB terminal T2, and output buffer 101, is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0228] Moreover, the start-up corrector circuit 35 so operates that,when the switching power supply apparatus shifts from start-up operationto steady-state operation, the resistor 39 b is connected in parallelwith the resistor 39 a to reduce the resistance between the FB terminalT2 of the IC 38 and the negative power supply line 2. This lowers thepotential at the FB terminal T2, and thereby ensures that the switchingpower supply apparatus performs reliable output voltage stabilizingcontrol when operating in the steady state.

[0229] Moreover, the signal level checker circuit 15, start-up correctorcircuit 35, and CS terminal controller circuit 37 can be realized with asimple circuit configuration, and the switching controller circuit canbe realized with an IC 38, i.e., FA5511. This helps reduce the space ofthe circuit board, and thereby reduce the size and cost of the switchingpower supply apparatus.

[0230] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation. Incidentally, with thecurrent technology, forming the main switching device in a singlepackage along with such other components results in giving the mainswitching device a high on-state resistance.

[0231] Sixth Embodiment

[0232]FIG. 10 is a circuit diagram of the switching power supplyapparatus of a sixth embodiment of the invention. In FIG. 10, suchcircuit components that find their counterparts in FIG. 9 are identifiedwith the same reference numerals, and their explanations will not berepeated.

[0233] In the switching power supply apparatus of the previousembodiment shown in FIG. 9, in burst switching operation, the period inwhich switching operation is stopped and the period in which switchingoperation is performed depend, as described earlier, on the delays inthe control performed by the output voltage control system. By contrast,in the switching power supply apparatus of this embodiment shown in FIG.10, the comparison reference power provided within the signal levelchecker circuit 15 a is varied between in the period in which switchingoperation is stopped and in the period in which switching operation isperformed so that, according to how the width of this variation is set,the period in which switching operation is stopped and the period inwhich switching operation is performed can be extended and adjusted.

[0234] In this embodiment, to permit the setting of the comparisonreference power, in the voltage division circuit provided in the signallevel checker circuit 15 shown in FIG. 9 and composed of the seriallyconnected resistors 50 and 51, the resistor 51 is divided into resistors51 a and 51 b as in the signal level checker circuit 15 a shown in FIG.10. The node between the resistors 51 a and 51 b is connected through adiode 59 to the CS terminal T8 of the IC 38.

[0235] In FIG. 10, when the switching power supply apparatus isoperating in the steady state, during the period in which switchingoperation is performed, the voltage at the CS terminal T8 of the IC 38is high, and the diode 59 prevents a current from flowing from the CStermina T8 to the node between the resistors 51 a and 51 b. Thus, thebase voltage of the transistor 48 (i.e., the comparison reference power)Esa is roughly set as given by formula (2) below.

Esa=[(Ra+Rb)×Ec]/(Ro+Ra+Rb)   (2)

[0236] In formula (2) above, Ra represents the resistance of theresistor 51 a, Rb represents the resistance of the resistor 51 b, Rorepresents the resistance of the resistor 50, and Ec represents thecharge voltage of the capacitor 33.

[0237] On the other hand, when the switching power supply apparatus isperforming burst switching operation, during the period in whichswitching operation is stopped, the transistor 53 is on, and theresistor 51 b is short-circuited. Thus, the base voltage of thetransistor 48 (i.e., the comparison reference power) Esb is roughly setas given by formula (3) below.

Esb=(Ra×Ec)/(Ro+Ra)   (3)

[0238] Hence, the relationship Esa>Esb holds. By appropriately settingthe resistances of the resistors 50, 51 a, and 51 b, it is possible tofeely set the value of Esa−Esb. While the switching power supplyapparatus is performing switching operation, when the output voltageincreases as a result of, for example, the load current decreasing, andthus the base voltage of the transistor 47 increases above the voltageEsa, as described above, a current is supplied from the transistor 48 tothe base of the transistor 53. This turns the transistor 53 on, and thusthe switching power supply apparatus stops switching operation.

[0239] As a result, the output voltage of the switching power supplyapparatus starts to decrease, and, when the base voltage of thetransistor 47 decreases below the voltage Esb, the transistor 48 turnsoff and turns the voltage at the CS terminal T8 of the IC 38 high. Thus,the switching power supply apparatus restarts switching operation. As aresult, the output voltage of the switching power supply apparatusincreases, and, when the base voltage of the transistor 47 increasesabove the voltage Esa, the switching power supply apparatus stopsswitching operation. This sequence of operations is repeated.

[0240] Accordingly, the switching power supply apparatus that adopts thesignal level checker circuit 15 a shown in FIG. 10 exhibits operationcharacteristics as described below.

[0241] In the signal level checker circuit 15 shown in FIG. 9, thevoltage of the comparison reference power is fixed. Thus, in a switchingpower supply apparatus adopting this signal level checker circuit 15,when it is performing burst switching operation, the lengths of theperiod in which switching is performed and the period in which switchingoperation is stopped depend on the delay characteristics of the controlperformed by the output voltage control system. By contrast, in aswitching power supply apparatus adopting the signal level checkercircuit 15 a shown in FIG. 10, the lengths of the period in whichswitching is performed and the period in which switching operation isstopped are longer than in the switching power supply apparatus adoptingthe signal level checker circuit 15, and in addition those lengths canbe freely set by appropriately setting the value of Esa−Esb as describedearlier.

[0242] Moreover, in a switching power supply apparatus adopting thesignal level checker circuit 15 shown in FIG. 9, when it is performingburst switching operation, the width of the fluctuation of the outputvoltage (the ripples in the output voltage) is set to be equal to themaximum value determined by the delay characteristics of the control ofthe output voltage control system. By contrast, in a switching powersupply apparatus adopting the signal level checker circuit 15 a shown inFIG. 10, the width of the fluctuation of the output voltage is greaterthan in the switching power supply apparatus adopting the signal levelchecker circuit 15, and in addition that width can be freely set byappropriately setting the value of Esa−Esb as described earlier.

[0243] Incidentally, increasing the width of the variation of the outputvoltage (i.e., the ripples in the output voltage) leads to the advantageof reducing the power loss suffered in burst switching operation.

[0244] Specifically, in a switching power supply apparatus adopting thesignal level checker circuit 15, switching operation is started when, inthe state in which switching operation is stopped, the signal level ofthe feedback signal decreases even slightly. At this time point at whichswitching operation is started, the voltage at the FB terminal T2 of theIC 38 increases little, and therefore the duty of the drive signaloutput from the output terminal T5 of the IC 38 is small (i.e., thehigh-level period is short).

[0245] By contrast, in a switching power supply apparatus adopting thesignal level checker circuit 15 a, switching operation is not starteduntil, in the state in which switching operation is stopped, the signallevel of the feedback signal decreases down to the level of the voltageEsb. Thus, at this time point at which switching operation is started,the voltage at the FB terminal T2 of the IC 38 increases greatly, andtherefore the duty of the drive signal output from the output terminalT5 is great (i.e., the high-level period is long).

[0246] Accordingly, at the time point at which switching operation isstarted, a strikingly large current per switching period is fed from thesecondary coil 6 of the transformer 3 through the diode 7, and thus,when observed in a long time span, switching has only to be performed asmaller number of times. This helps reduce power loss.

[0247] Therefore, in applications where a minimum fluctuation width ispermitted in the output voltage in burst switching operation, the signallevel checker circuit 15 shown in FIG. 9 is adopted, and, inapplications where priority is given to reduction of power consumption,the signal level checker circuit 15 a shown in FIG. 10 is adopted. In acase where the signal level checker circuit 15 a is adopted, asdescribed earlier, the width of the fluctuation of the output voltagecan be set to be the optimum value that produces fluctuation smallerthan applications permit and that simultaneously minimizes powerconsumption.

[0248] Incidentally, when a switching power supply apparatus adoptingthe signal level checker circuit 15 a is operating in a heavy-loadstate, its output voltage tends to decrease. This keeps the base voltageof the transistor 47 lower than the voltage Esa, and thus the switchingpower supply apparatus performs continuous switching.

[0249] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 a achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe IC 38 is also stopped. This helps reduce the power loss sufferedwhile the switching operation is being stopped, and thus helps reducethe power consumption of the apparatus as a whole.

[0250] Moreover, the start-up corrector circuit 35 so operates that,when the switching power supply apparatus shifts from start-up operationto steady-state operation, the resistor 39 b is connected in parallelwith the resistor 39 a to reduce the resistance between the FB terminalT2 of the IC 38 and the negative power supply line 2. This lowers thepotential at the FB terminal T2, and thereby ensures that the switchingpower supply apparatus performs reliable output voltage stabilizingcontrol when operating in the steady state.

[0251] Moreover, the signal level checker circuit 15 a, start-upcorrector circuit 35, and CS terminal controller circuit 37 can berealized with a simple circuit configuration, and the switchingcontroller circuit can be realized with an IC 38, i.e., FA5511. Thishelps reduce the space of the circuit board, and thereby reduce the sizeand cost of the switching power supply apparatus.

[0252] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation. Incidentally, with thecurrent technology, forming the main switching device in a singlepackage along with such other components results in giving the mainswitching device a high on-state resistance.

[0253] Seventh Embodiment

[0254]FIG. 11 is a circuit diagram of the switching power supplyapparatus of a seventh embodiment of the invention. In FIG. 11, suchcircuit components that find their counterparts in FIG. 4 are identifiedwith the same reference numerals, and their explanations will not berepeated.

[0255] In the switching power supply apparatus shown in FIG. 4, thefeedback signal is fed from the phototransistor 20 b through the signallevel checker circuit 15 and then, after having the increase or decreasein its signal level inverted as described earlier, to the FB terminal T2of the IC 38. By contrast, in the switching power supply apparatus ofthis embodiment shown in FIG. 11, the feedback signal is fed from thephototransistor 20 b through the resistor 34 and a current adjustercircuit 60 to the FB terminal T2 of the IC 38. Moreover, in thisembodiment, the start-up corrector circuit is omitted for the reasonstated later.

[0256] The current adjuster circuit 60 absorbs from the FB terminal T2of the IC 38 a current proportional to the voltage at the node betweenthe phototransistor 20 b and the resistor 34. Accordingly, when theoutput voltage of the switching power supply apparatus is, for example,higher than a predetermined value, the output voltage detector circuit 9increases the voltage at the node between the phototransistor 20 b andthe resistor 34, and the current adjuster circuit 60 increases, in amanner corresponding to the increase in that voltage, the current thatit absorbs from the FB terminal T2 of the IC 38. This causes the voltageat the FB terminal T2 to decrease.

[0257] As this voltage decreases, the PWM logic circuit 105 (see FIG. 6)provided within the IC 38 feeds, via the output terminal T5 of the IC38, the main switching device 5 with a drive signal of which thehigh-level period is short. This causes the current supplied from thesecondary coil 6 of the transformer 3 through the diode 7 to decrease,and thus the output voltage is so controlled as to decrease.

[0258] On the other hand, when the output voltage of the switching powersupply apparatus is, for example, lower than the predetermined value,the output voltage detector circuit 9 decreases the voltage at the nodebetween the phototransistor 20 b and the resistor 34, and the currentadjuster circuit 60 decreases, in a manner corresponding to the decreasein that voltage, the current that it absorbs from the FB terminal T2 ofthe IC 38. This causes the voltage at the FB terminal T2 to increase.

[0259] As this voltage increases, the PWM logic circuit 105 (see FIG. 6)provided within the IC 38 feeds, via the output terminal T5 of the IC38, the main switching device 5 with a drive signal of which thehigh-level period is long. This causes the current supplied from thesecondary coil 6 of the transformer 3 through the diode 7 to increase,and thus the output voltage is so controlled as to increase.

[0260] The principle on which the switching power supply apparatus shownin FIG. 11 achieves burst switching control is the same as that on whichthe switching power supply apparatus shown in FIG. 4 achieves burstswitching control.

[0261] The switching power supply apparatus of this embodiment, atstart-up, starts up in the same manner as a common circuit (see FIG. 5)that employs FA5511 (IC 38). This makes it possible to omit the start-upcorrector circuit described earlier.

[0262] Specifically, at the time point t1 shown in FIG. 7, the chargevoltage of the capacitor 33 is zero, the voltage at the node between thephototransistor 20 b and the resistor 34 is also zero, and the currentadjuster circuit 60 does not absorb a current from the FB terminal T2 ofthe IC 38. Thus, the voltage at the FB terminal T2 has the same level asthe output voltage of the 5 V voltage regulator 103 (see FIG. 6).

[0263] Thereafter, the output voltage of the switching power supplyapparatus and the charge voltage of the capacitor 33 increase, and,when, at the time point t3, the output voltage of the switching powersupply apparatus reaches close to the predetermined voltage set by theresistors 23 and 24, a current flows through the phototransistor 20 b.This causes the voltage at the node between the phototransistor 20 b andthe resistor 34 to increase, and the voltage at the FB terminal T2 ofthe IC 38 start to decrease. Now, control for outputting a stabilizedsteady-state voltage is started. During the period up to the time pointt3, the duty of the drive signal output from the output terminal T5 ofthe IC 38 is controlled by the voltage level at the CS terminal T8.

[0264] The operation described above is the same as the operationperformed at start-up by a common circuit employing FA5511 like the onedescribed in connection with the switching power supply apparatus shownin FIG. 4. The switching power supply apparatus shown in FIG. 11 doesnot require a start-up corrector circuit.

[0265] As described above, the switching power supply apparatus shown inFIG. 11 does not require a start-up corrector circuit, and thus has anaccordingly simpler circuit configuration. However, in this switchingpower supply apparatus, as will be described later in connection withthe eighth embodiment, the characteristics of the individual componentssuch as semiconductor devices used in the current adjuster circuit 60drift with temperature. Disadvantageously, this causes variation in theload current target value at which switching is performed from burstswitching to normal continuous switching and the load current targetvalue at which switching is performed from normal continuous switchingto burst switching. Therefore, the switching power supply apparatusshown in FIG. 11 is suitable in applications where variation in the loadcurrent target—values is permitted.

[0266] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit.Moreover, in burst switching control, while the switching operation ofthe main switching device 5 is being stopped, the supply of operatingpower to the principal components of the IC 38, namely the OSC 106, PWMlogic circuit 105, FB terminal T2, and output buffer 101, is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0267] Moreover, when the switching power supply apparatus starts up,the current adjuster circuit 60 so operates as to adjust the current atthe FB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

[0268] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation.

[0269] Eighth Embodiment

[0270]FIG. 12 is a circuit diagram of the switching power supplyapparatus of an eighth embodiment of the invention. In FIG. 12, suchcircuit components that find their counterparts in FIGS. 10 and 11 areidentified with the same reference numerals, and their explanations willnot be repeated.

[0271] In FIG. 12, the current adjuster circuit 60 is composed of atransistor 70 and a resistor 72. The transistor 70 has its collectorconnected to the FB terminal T2 of the IC 38, has its base connected tothe node between the resistor 34 and a resistor 71, and has its emitterconnected through the resistor 72 to the negative power supply line 2.

[0272] When the switching power supply apparatus is performing normalcontinuous switching operation, the voltage Ef at the FB terminal T2 ofthe IC 38 is roughly determined by formula (4) below.

Ef=Er−(Ea−Vb)×Re/Rc−Vf   (4)

[0273] In formula (4) above, Er represents the output voltage of the 5 Vvoltage regulator 103 (see FIG. 6) provided within the IC 38, i.e.,FA5511, Ea represents the voltage at the node between the resistor 71and the resistor 34, Vb represents the forward voltage between the baseand emitter of the transistor 70, Re represents the resistance of thepull-up resistor 108 (see FIG. 6) provided within the IC 38, Rcrepresents the resistance of the resistor 72, and Vf represents theforward voltage drop across the diode 107 (see FIG. 6) provided withinthe IC 38.

[0274] As will be clear from formula (4) above, the voltage Ef relatesto the forward voltage between the base and emitter of the transistor70. In general, the forward voltage between the base and emitter of atransistor varies with temperature. Therefore, even when the basevoltage of the transistor 47 is stable, as the operating ambienttemperature varies, the forward voltage of the transistor 70 varies, andthus the voltage at the FB terminal T2 of the IC 38 varies.

[0275] Moreover, when the switching power supply apparatus is performingcontinuous switching, as described earlier, the voltage level at the FBterminal T2 of the IC 38 is varied according to the variation of theoutput voltage of the switching power supply apparatus so that theoutput voltage is stabilized. This means, since the output voltagedepends on the variation of the load, that the output voltage isstabilized by varying the voltage level at the FB terminal T2 accordingto the variation of the load current. Thus, the voltage value at the FBterminal T2 represents the load current of the switching power supplyapparatus.

[0276] In the switching power supply apparatus, as described earlier, incontinuous switching operation, as the load current is decreasedgradually, the base voltage of the transistor 47 increases, and, when itbecomes higher than the base voltage (comparison reference voltage) ofthe transistor 48, the transistor 53 turns on, achieving a shift intoburst switching operation. Thus, if the voltage at the FB terminal T2 ofthe IC 38 at the time point of this switching varies according to theoperating ambient temperature of the switching power supply apparatus,it follows that, quite undesirably, the load current at the time pointof that switching also varies according to the operating ambienttemperature of the switching power supply apparatus.

[0277] Depending on the type of the appliance connected to the switchingpower supply apparatus, the appliance may require an accurate value asthe reference relative to which to evaluate the load current todetermine whether to perform switching or not. Thus, in suchapplications, the switching power supply apparatus is not very suitable.However, in applications where such accuracy is not required, theswitching power supply apparatus, having a comparatively simpleconfiguration, is suitable.

[0278] Incidentally, in the circuits shown in FIGS. 4, 9, and 10described earlier, instead of providing a current adjuster circuit 60,the causes for the variation of the load current are eliminated. Thismakes it possible to comparatively accurately evaluate the load currentat the time of operation mode switching. However, the additionalprovision of the start-up corrector circuit 35 makes the circuitconfiguration a little more complicated.

[0279] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal circuit portions of the IC 38, namely the OSC 106, PWMlogic circuit 105, FB terminal T2, and output buffer 101, is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0280] Moreover, when the switching power supply apparatus starts up,the current adjuster circuit 60 so operates as to adjust the current atthe FB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

[0281] Moreover, the signal level checker circuit 15, current adjustercircuit 60, and CS terminal controller circuit 37 can be realized with asimple circuit configuration, and the switching controller circuit canbe realized with an IC 38, i.e., FA5511. This helps reduce the space ofthe circuit board, and thereby reduce the size and cost of the switchingpower supply apparatus.

[0282] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation.

[0283] Ninth Embodiment

[0284]FIG. 13 is a circuit diagram of the switching power supplyapparatus of a ninth embodiment of the invention. In FIG. 13, suchcircuit components that find their counterparts in FIG. 12 areidentified with the same reference numerals, and their explanations willnot be repeated.

[0285] The circuit shown in FIG. 13 differs from that shown in FIG. 12in that a transistor 77 having identical characteristics with thetransistor 70 is additionally connected in series with the resistor 71.By the action of this transistor 77, the circuit shown in FIG. 13 canalleviate drift of characteristics with temperature. Specifically, forexample, when the operating ambient temperature of the switching powersupply apparatus rises, at the same time that the forward voltagebetween the base and emitter of the transistor 70 decreases, the forwardvoltage between the base and emitter of the transistor 77 alsodecreases. This causes the base voltage of the transistor 70 todecrease, and thereby suppresses variation of the voltage at the FBterminal T2 of the IC 38.

[0286] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe IC 38 is also stopped. This helps reduce the power loss sufferedwhile the switching operation is being stopped, and thus helps reducethe power consumption of the apparatus as a whole.

[0287] Moreover, when the switching power supply apparatus starts up,the current adjuster circuit 60 so operates as to adjust the current atthe FB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

[0288] Moreover, the signal level checker circuit 15, current adjustercircuit 60, and CS terminal controller circuit 37 can be realized with asimple circuit configuration, and the switching controller circuit canbe realized with an IC 38, i.e., FA5511. This helps reduce the space ofthe circuit board, and thereby reduce the size and cost of the switchingpower supply apparatus.

[0289] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation.

[0290] Tenth Embodiment

[0291]FIG. 14 is a circuit diagram of the switching power supplyapparatus of a tenth embodiment of the invention. In FIG. 14, suchcircuit components that find their counterparts in FIG. 11 areidentified with the same reference numerals, and their explanations willnot be repeated.

[0292] The circuit shown in FIG. 14 differs from that shown in FIG. 11in that a capacitor 75 is additionally connected between the terminal T7and the FB terminal T2 of the IC 38. Moreover, in the circuit shown inFIG. 14, mainly for phase compensation of the output stabilizing controlsystem in the continuous switching state, a serial circuit composed of aresistor 73 and a capacitor 74 may be additionally connected between theFB terminal T2 of the IC 38 and the negative power supply line 2.

[0293] Adding the capacitor 74 and the resistor 73, however, causes thefollowing undesirable phenomenon in burst switching operation.

[0294] When the switching power supply apparatus is performing burstswitching operation, during the period in which switching operation isstopped, as described earlier, the output voltage of the 5 V voltageregulator 103 (see FIG. 6) is zero, and thus the voltage at the FBterminal T2 of the IC 38 decreases. Thereafter, as described earlier, atthe time point at which switching operation is started, the outputvoltage of the 5 V voltage regulator 103 rises, and thus a current flowsinto the capacitor 74 through the diode 107 (see FIG. 6) and theresistor 108 (see FIG. 6). Thus, it takes a while for the voltage at theFB terminal T2 of the IC 38 to reach the lower-limit voltage level ofthe output signal of the OSC 106. This delays the starting of switchingoperation.

[0295] For example, while the switching power supply apparatus isperforming burst switching operation, when the load current abruptlyincreases during the period in which switching operation is stopped, thedecrease in the output voltage of the switching power supply apparatusis detected by the signal level checker circuit 15 detecting a decreasein the feedback signal. In this case, even if the output voltage of the5 V voltage regulator 103 (FIG. 6) is made to rise quickly, theaforementioned delay in the starting of switching operation, quitedisadvantageously, lets the output voltage of the switching power supplyapparatus further decrease during the delay.

[0296] That is, in burst switching operation, when the load increasesabruptly, the delay in the operation of the burst switching operationcontrol system increases the amount by which the output voltage of theswitching power supply apparatus decreases. For this reason, it isdesirable to increase the control speed of the burst switching controlsystem as much as possible.

[0297] Incidentally, when the output of the 5 V voltage regulator 103(see FIG. 6) rises, by supplying a current through the capacitor 75 tothe capacitor 74, it is possible to eliminate the delay of the operationof the burst switching operation control system. Moreover, by giving thecapacitor 75 a capacitance higher than that required to eliminate thedelay of the operation of the burst switching operation control system,it is possible to achieve the same effects as those achieved in theembodiment shown in FIG. 10.

[0298] Specifically, when the capacitor 75 is given so high acapacitance, in burst switching operation, at the time point at whichswitching operation is started, the voltage at the FB terminal T2 of theIC 38 becomes higher than the value corresponding to the level of thefeedback signal, and thus a drive signal having a great duty is fed viathe output terminal T5 of the IC 38 to the main switching device 5.Thus, for the same reasons as stated in connection with the embodimentshown in FIG. 10, the switching power supply apparatus shown in FIG. 14contributes to reduction of the power loss suffered in the burstswitching operation.

[0299] As compared with the switching power supply apparatus of theembodiment shown in FIG. 10, however, the way how power loss in burstswitching operation is reduced in the switching power supply apparatusof the embodiment shown in FIG. 14 is a little less reliable because ofthe difficulty in setting the capacitance of the added capacitor 75.That is, in the switching power supply apparatus of the embodiment shownin FIG. 14, the addition of the capacitor 75 affects the phasecompensation of the output voltage stabilizing control system, andtherefore this configuration can be suitably adopted in a case where,with the capacitor 75 added, the desired phase compensation is achieved.

[0300] Incidentally, the addition of the capacitor 75 is effective alsoin a case where the capacitor 74 and the resistor 73 are added in theswitching power supply apparatus of the embodiment shown in FIGS. 4, 16,or 19.

[0301] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal components of the IC 38, namely the OSC 106, PWM logiccircuit 105, FB terminal T2, and output buffer 101, is also stopped.This helps reduce the power loss suffered while the switching operationis being stopped, and thus helps reduce the power consumption of theapparatus as a whole.

[0302] Moreover, when the switching power supply apparatus starts up,the current adjuster circuit 60 so operates as to adjust the current atthe FB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

[0303] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation.

[0304] Eleventh Embodiment

[0305]FIG. 15 is a circuit diagram of the switching power supplyapparatus of an eleventh embodiment of the invention. In FIG. 15, suchcircuit components that find their counterparts in FIG. 14 areidentified with the same reference numerals, and their explanations willnot be repeated.

[0306] The switching power supply apparatus shown in FIG. 15 differsfrom that shown in FIG. 14 in that the phase compensation circuitcomposed of the capacitor 74 and the resistor 73 which is provided inthe latter is replaced with two serial circuits in the former,specifically one composed of a capacitor 75 and a resistor 76 andanother composed of a capacitor 78 and a resistor 79 as shown in FIG.15. By giving these capacitors 75 and 78 and resistors 76 and 69capacitances and resistances that fulfill formulae (5) and (6) below, itis possible to completely eliminate the effects of the phasecompensation circuit in burst switching operation and to realize thedesired phase compensation in continuous switching operation.

Ca×Rm=Cb×Rn   (5)

Ed=Er×Ca/(Ca+Cb)   (6)

[0307] In formulae (5) and (6) above, Ca represents the capacitance ofthe capacitor 75, Cb represents the capacitance of the capacitor 78, Rmrepresents the resistance of the resistor 76, Rn represents theresistance of the resistor 79, Er represents the output voltage of the 5V voltage regulator 103 (see FIG. 6), and Ed represents the voltage drop(variation in voltage) that occurs at the FB terminal T2 of the IC 38.

[0308] More specifically, Ed represents the voltage drop (variation involtage) that occurs at the FB terminal T2 of the IC 38 when the 5 Vvoltage regulator 103 stops its output in burst switching operation. Forexample, in the switching power supply apparatus of the embodiment shownin FIG. 15, immediately before the 5 V voltage regulator 103 stops itsoutput, the current adjuster circuit 60 absorbs from the FB terminal T2a current commensurate with the signal level of the feedback signal fedfrom the phototransistor 20 b, and thus the voltage at the FB terminalT2 is kept at the voltage level commensurate with the absorbed current.However, as soon as the 5 V voltage regulator 103 stops its output, thevoltage at the FB terminal T2 drops to zero. Ed represents this voltagedifference (voltage drop).

[0309] In formula (6), if the value of the right side is made greaterthan Ed, in burst switching operation, at the time point at whichswitching operation is started, the voltage at the FB terminal T2becomes higher than the value corresponding to the level of the feedbacksignal, and thus a drive signal having a great duty is fed via theoutput terminal T5 of the IC 38 to the main switching device 5. Thus,for the same reasons as stated in connection with the embodiment shownin FIG. 10, the switching power supply apparatus shown in FIG. 15contributes to reduction of the power loss suffered in the burstswitching operation.

[0310] Moreover, in the switching power supply apparatus of theembodiment shown in FIG. 15, by setting the capacitances of thecapacitors and the resistances of the resistors in such a way that theyfulfill formulae (7) and (8) below, it is possible, in continuousswitching operation, to obtain the same phase compensationcharacteristic as when phase compensation is achieved with the serialcircuit composed of the capacitor 74 and the resistor 73 (see FIG. 14)alone.

Ca×Rm=Cb×Rn=Cd×Rt   (7)

Cd=Ca+Cb   (8)

[0311] In formulae (7) and (8) above, Ca represents the capacitance ofthe capacitor 75, Cb represents the capacitance of the capacitor 78, Cdrepresents the capacitance of the capacitor 74 (see FIG. 14), Rmrepresents the resistance of the resistor 76, Rn represents theresistance of the resistor 79, and Rt represents the resistance of theresistor 73.

[0312] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe IC 38 is also stopped. This helps reduce the power loss sufferedwhile the switching operation is being stopped, and thus helps reducethe power consumption of the apparatus as a whole.

[0313] Moreover, when the switching power supply apparatus starts up,the current adjuster circuit 60 so operates as to adjust the current atthe FB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

[0314] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation.

[0315] Twelfth Embodiment

[0316]FIG. 16 is a circuit diagram of the switching power supplyapparatus of a twelfth embodiment of the invention. In FIG. 16, suchcircuit components that find their counterparts in FIG. 4 are identifiedwith the same reference numerals, and their explanations will not berepeated.

[0317] In the switching power supply apparatus of this embodiment shownin FIG. 16, the subsidiary control power used in FIG. 4 is omitted, andinstead a direct current produced by rectifying with the diode 31 thevoltage induced in the subsidiary coil 32 of the transformer 3 is usedas the power of the control circuit, and is thus fed directly to thecapacitor 46.

[0318] When the switching power supply apparatus shown in FIG. 16 startsto start up, as described earlier, the start-up current that is suppliedthrough the start-up resistor 29 flows through the signal level checkercircuit 15, and this lengthens the time required for the charge voltageof the capacitor 46 to reach the operation starting voltage of the IC38, i.e., FA5511. To prevent this, here, a start-up switcher circuit 81is additionally provided.

[0319] The output current (feedback signal) of the phototransistor 20 bis fed through a diode 80 to the signal level checker circuit 15, andthe start-up switcher circuit 81 and a start-up corrector circuit 82check whether the feedback signal is present or not by monitoring thevoltage at the node between the phototransistor 20 b and the diode 80.

[0320] The current consumed by the signal level checker circuit 15 (thecurrent consumed including that consumed by the comparison referencepower) is fed thereto from the positive terminal of the capacitor 46 byway of a line 84, and is returned by way of a line 83 through the switchprovided within the start-up switcher circuit 81 to the negativeterminal of the capacitor 46. On the other hand, the current through thephototransistor 20 b is fed thereto from the positive terminal of thecapacitor 46, and is returned through the diode 80, thecurrent-detection resistor 34, and the switch provided within thestart-up switcher circuit 81 to the negative terminal of the capacitor46.

[0321] When the switching power supply apparatus starts to start up, theinternal switch of the start-up switcher circuit 81 and the internalswitch of the start-up corrector circuit 82 are off, and the outputvoltage of the switching power supply apparatus is lower than apredetermined target voltage. Thus, no current is consumed by the signallevel checker circuit 15 (including the comparison reference powerprovided therein) or the phototransistor 20 b. Accordingly, the chargevoltage of the capacitor 46, owing to the start-up current fed theretothrough the start-up resistor 29, quickly rises and reaches theoperation start voltage level of the IC 38, i.e., FA5511. The timerequired for the charge voltage of the capacitor 46 to rise here isroughly as short as in a common configuration employing FF5511.

[0322] Next, the start-up operation of the switching power supplyapparatus will be described with reference to a signal waveform diagramshown in FIG. 17.

[0323] When, at a time point A0 shown at (a) in FIG. 17, adirect-current voltage is applied between the positive and negativepower supply lines 1 and 2, the voltage 213 across the capacitor 46increases gradually owing to the charge current supplied thereto throughthe start-up resistor 29. When, at a time point Al, the voltage 213across the capacitor 46 reaches the predetermined operation startingvoltage of the IC 38, i.e., FA5511, the voltage on the internal supplyline 104 within the IC 38 rises, and thus the OSC 106, PWM logic circuit105, and output buffer 101 starts to operate.

[0324] Thus, the OSC 106 feeds the PWM logic circuit 105 with anoscillation signal 214 having constant upper and lower limits and aconstant period, and accordingly the voltage 216 at the CS terminal T8of the IC 38 increases gradually.

[0325] Moreover, at the time point Al, as described above, the start-upswitch 81 is off, and therefore the signal level checker circuit 15 isnot supplied with operating current. Thus, the output current of thesignal level checker circuit 15 is zero. Moreover, the switch of thestart-up corrector circuit 82 is off, and thus the voltage 215 at the FBterminal T2 of the IC 38 is equal to the division voltage resulting fromvoltage division by the diode 107 (see FIG. 6) provided within the IC38, the resistor 108 (see FIG. 6), and the resistor 39 a (see FIG. 16).Here, the resistance of the resistor 39 a is so set that this divisionvoltage has roughly the same level as the upper-limit voltage level ofthe oscillation signal 214 of the OSC 106.

[0326] As described earlier, when whichever of the voltage 216 at the CSterminal T8 of the IC 38 and the voltage 215 at the FB terminal T2 islower is higher than the voltage level of the oscillation signal 214 ofthe OSC 106, the PWM logic circuit 105 outputs a high-level voltage viathe output terminal T5.

[0327] Accordingly, as shown at (c) in FIG. 17, during the period fromthe time point A0 to a time point A2, during which the level of thevoltage 216 at the CS terminal T8 of the IC 38 is lower than the voltagelevel of the oscillation signal 214 of the OSC 106, the voltage 217 atthe output terminal T5 of the IC 38 remains low. At the time point A2,when the voltage level of the voltage 216 at the CS terminal T8momentarily exceeds the voltage level of the oscillation signal 214 ofthe OSC 106, the voltage 217 at the output terminal T5 becomes high andthen remains high for the corresponding period, turning the mainswitching device 5 on.

[0328] When the main switching device 5 is turned on in this way, thevoltage between the output lines 25 and 26 slightly increases, and,during the period up to a time point A3, as the voltage 216 at the CSterminal T8 of the IC 38 increases, the duty of the drive signal 217output via the output terminal T5 of the IC 38 continues to increase.This causes the output voltage of the switching power supply apparatusto increase quickly.

[0329] When, at a time point A3, the output voltage of the switchingpower supply apparatus reaches close to the predetermined targetvoltage, (i.e., when the voltage resulting from voltage division of theoutput voltage by the resistors 23 and 24 reaches a level roughly equalto the comparison reference voltage within the shunt regulator 22), acurrent flows through the shunt regulator 22 and the photodiode 20 a,and thus the voltage at the node between the phototransistor 20 b andthe diode 80 increases. On detecting the increase in this voltage, thestart-up switcher circuit 81 turns its internal switch on to permit acurrent to flow through the signal level checker circuit 15 and theresistor 34. This causes the relevant circuits to start to operate.

[0330] On the other hand, the charge voltage of the capacitor 46, owingto the current supplied thereto from the subsidiary coil 32 of thetransformer 3 through the diode 31, starts to increase immediatelybefore the time point A3 at which the output voltage of the switchingpower supply apparatus reaches the predetermined target voltage. At thetime point A3, the charge voltage of the capacitor 46 has reached thevalue determined by the predetermined target output voltage of theswitching power supply apparatus and the winding ratio between thesubsidiary coil 32 and secondary coil 6 of the transformer 3. Thus, thecurrent flowing through the signal level checker circuit 15 and theresistor 34 prevents the operating voltage of the IC 38 from fallingbelow the permitted minimum operating voltage and thereby prevents itsmalfunctioning.

[0331] At the time point A3, the start-up corrector circuit 82 turns itsinternal switch on as does the start-up switcher circuit 81, and thusthe resistor 39 b is connected in parallel with the resistor 39 a. Now,as will be described later, the voltage 215 at the FB terminal T2 of theIC 38 starts to perform steady-state operation. It should be noted thatFIG. 17 illustrates an example in which the switching power supplyapparatus starts up in a heavy-load state, including what is shown inFIG. 8 described earlier.

[0332] When the current value through the phototransistor 20 b is lowerthan a predetermined value set within the signal level checker circuit15, the signal level checker circuit 15 supplies a current to the FBterminal T2; by contrast, when the current value through thephototransistor 20 b is higher than the predetermined value set withinthe signal level checker circuit 15, the signal level checker circuit 15feeds a current to the CS terminal controller circuit 37 to turn theinternal switch of the CS terminal controller circuit 37 on, and stopsthe supply of operating power to the principal circuit portions of theIC 38, namely the OSC 106, PWM logic circuit 105, FB terminal T2, andoutput buffer 101.

[0333] Incidentally, the signal level checker circuit 15 does not supplya current simultaneously to the FB terminal T2 and the CS terminalcontroller circuit 37.

[0334] Moreover, while the signal level checker circuit 15 is supplyinga current to the FB terminal T2 of the IC 38, the signal level checkercircuit 15 so functions as to decrease the supply current when thecurrent value through the phototransistor 20 b increases and increasethe supply current when the current value through the phototransistor 20b decreases. This function permits the output voltage of the switchingpower supply apparatus to be stabilized at the predetermined targetvalue.

[0335] Incidentally, the start-up corrector circuit 82 switches theresistance between the FB terminal T2 and the negative power supply line2 between when the switching power supply apparatus is starting up andwhen it is operating in the steady state. This ensures that theswitching power supply apparatus operates reliably.

[0336] Specifically, when the switching power supply apparatus starts tostart up, the signal level checker circuit 15 is not operating, and thusno current is supplied from the signal level checker circuit 15.Therefore, the resistor 39 a is given a high resistance so that thevoltage resulting from voltage division by the diode 107 provided withinthe IC 38, i.e., FA5511, the resistor 108 (see FIG. 6), and the resistor39 a (see FIG. 16) is close to the upper limit of the oscillation signal214 of the OSC 106. If this is not the case, even after the voltagelevel 216 at the CS terminal T8 of the IC 38 increases, the voltagelevel 215 at the FB terminal T2 remains lower than the lower limit ofthe oscillation signal of the OSC 106, and thus the PWM logic circuit105 (see FIG. 6) does not output a high-level signal via the outputterminal T5. This makes it impossible for the output voltage of theswitching power supply apparatus to rise.

[0337] On the other hand, in steady-state operation, if the resistancebetween the FB terminal T2 and the negative power supply line 2 is kepthigh, for example, when the output voltage of the switching power supplyapparatus increases as a result of the switching power supply apparatusoperating in a no-load state, as the output voltage is stabilized in themanner described earlier, even when the signal level checker circuit 15stops the supply current, the voltage at the FB terminal T2 does notfall below the lower-limit voltage of the oscillation signal 214 of theOSC 106 owing to the current supplied from the output line 104 of the 5V voltage regulator 103 (see FIG. 6) through the diode 107 and theresistor 108. Quite inconveniently, this makes it impossible to decreasethe output voltage of the switching power supply apparatus.

[0338] To overcome this inconvenience, when the switching power supplyapparatus starts up, at the time point A3, the start-up correctorcircuit 82 reduces the resistance between the FB terminal T2 and thenegative power supply line 2. After the time point A3, the internalswitches of the start-up switcher circuit 81 and the start-up correctorcircuit 82 are kept on, and thus, in light-load operation, the switchingpower supply apparatus performs burst switching operation on the sameprinciple as described in connection with the embodiment shown in FIG.4. This helps reduce power loss in light-load operation.

[0339] The switching power supply apparatus of this embodiment shown inFIG. 16 permits omission of the capacitor 33 and the diode 30 used inthe switching power supply apparatus shown in FIG. 4, but insteadrequires addition of the start-up switcher circuit 81. While in thisembodiment the start-up switcher circuit 81 can easily be incorporatedin an IC, the capacitor 33 cannot be incorporated in an IC. Thus, thisembodiment is suitable to produce a new IC incorporating FA5511 or anequivalent IC along with a CS terminal controller circuit, signal levelchecker circuit, start-up switcher circuit, start-up corrector circuit,and other attendant circuits.

[0340] This embodiment is more susceptible than the embodiment shown inFIG. 4 to the effects of temperature-related drift of the forwardvoltage of the diode 80, resulting in the disadvantage of a small degreeof temperature-related drift of the load current target value at whichswitching between burst switching and continuous switching is performed.Therefore, it is advisable to adopt the embodiment shown in FIG. 4 inapplications where the effects of temperature-related drift needs to bestrictly eliminated, and adopt the circuit of this embodiment inapplications where no such strict requirement needs to be met.

[0341] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal circuit portions of the IC 38, namely the OSC 106, PWMlogic circuit 105, FB terminal T2, and output buffer 101, is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0342] Moreover, the start-up corrector circuit 35 so operates that thesecond resistor 39 b is connected in parallel with the first resistor 39a to reduce the resistance between the FB terminal T2 of the IC 38 andthe negative power supply line 2. This considerably lowers the potentialat the FB terminal T2, and thus the IC 38 can make the main switchingdevice 5 perform switching operation quickly. This helps reduce start-uptime.

[0343] Moreover, through the operation of the start-up switcher circuit81, when the switching power supply apparatus starts to start up, thestart-up current supplied through the start-up resistor 29 is preventedfrom flowing through the signal level checker circuit 15 and therebylengthening the time required for the charge voltage of the capacitor 46to reach the operation starting voltage of the IC 38, i.e., FA5111.

[0344] Thirteenth Embodiment

[0345]FIG. 18 is a circuit diagram of the switching power supplyapparatus of a thirteenth embodiment of the invention. In FIG. 18, suchcircuit components that find their counterparts in FIGS. 9 and 16 areidentified with the same reference numerals, and their explanations willnot be repeated.

[0346] In FIG. 18, the signal level checker circuit 15 is composed ofresistors 49, 50, and 51 and transistors 47 and 48 The CS terminalcontroller circuit 37 is composed of a resistor 52 and a transistor 53.The start-up switcher circuit 81 is composed of a resistor 85 and atransistor 84. The start-up corrector circuit 82 is composed ofresistors 87 and 39 b and a transistor 86.

[0347] As shown in FIG. 17 described earlier, when the switching powersupply apparatus starts to start up, at the time point A3, a currentstarts to flow through the phototransistor 20 b, and the voltage at thenode between the emitter of the phototransistor 20 b and the diode 80increases. This voltage is fed through the resistor 85 to the base ofthe transistor 84 and through the resistor 87 to the base of thetransistor 86, and thus the transistors 84 and 86 turn on. As the resultof the transistor 84 turning on, a current flows through the resistor 34and through the serial circuit composed of the resistors 50 and 51, anda base current starts to flow through the transistor 47. Thus, thesignal level checker circuit 15 starts to operate.

[0348] As described earlier, during the period up to the time point A3,no current flows through the signal level checker circuit 15. Thisprevents the lengthening of the time required for the charge voltage ofthe capacitor 46 to reach the operation starting voltage of the IC 38,i.e., FA5511.

[0349] Moreover, as the result of the transistor 86 turning on, theresistor 39 b is added between the FB terminal T2 of the IC 38 and thenegative power supply line 2. This ensures that the switching powersupply apparatus performs reliable output voltage stabilizing controlwhen operating in the steady state.

[0350] Incidentally, during the period up to the time point A3, thediode 80 prevents the base currents of the transistors 47, 84, and 86from flowing along the route from the positive terminal of the capacitor46 to the resistor 49, to the emitter of the transistor 47, to the baseof the transistor 47, to the resistor 85, to the base of the transistor84, to the emitter of the transistor 84, to the negative power supplyline 2, and to the negative terminal of the capacitor 46 or along theroute from the positive terminal of the capacitor 46 to the resistor 49,to the emitter of the transistor 47, to the base of the transistor 47,to the resistor 87, to the base of the transistor 86, to the emitter ofthe transistor 86, to the negative power supply line 2, and to thenegative terminal of the capacitor 46. Thus, that this period, the diode80 prevents the transistors 47, 84, and 86 from being turned on, andthereby prevents the signal level checker circuit 15 from operating andthe resistor 39 b from being connected between the FB terminal T2 of theIC 38 and the negative power supply line 2.

[0351] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal circuit portions of the IC 38, namely the OSC 106, PWMlogic circuit 105, FB terminal T2, and output buffer 101, is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0352] Moreover, the start-up corrector circuit 82 so operates that,when the switching power supply apparatus shifts from start-up operationto steady-state operation, the second resistor 39 b is connected inparallel with the first resistor 39 a to reduce the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2.This lowers the potential at the FB terminal T2, and thereby ensuresthat the switching power supply apparatus performs reliable outputvoltage stabilizing control when operating in the steady state.

[0353] Moreover, through the operation of the start-up switcher circuit81, when the switching power supply apparatus starts to start up, thestart-up current supplied through the start-up resistor 29 is preventedfrom flowing through the signal level checker circuit 15 and therebylengthening the time required for the charge voltage of the capacitor 46to reach the operation starting voltage of the IC 38, i.e., FA5111.

[0354] Moreover, the signal level checker circuit 15, start-up switchercircuit 81, start-up corrector circuit 82, and CS terminal controllercircuit 37 can be realized with a simple circuit configuration, and theswitching controller circuit can be realized with an IC 38, i.e.,FA5511. This helps reduce the space of the circuit board, and therebyreduce the size and cost of the switching power supply apparatus.

[0355] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation.

[0356] Fourteenth Embodiment

[0357]FIG. 19 is a circuit diagram of the switching power supplyapparatus of a fourteenth embodiment of the invention. In FIG. 19, suchcircuit components that find their counterparts in FIG. 16 areidentified with the same reference numerals, and their explanations willnot be repeated.

[0358] The switching power supply apparatus shown in FIG. 19 differsfrom that shown in FIG. 16 in that the start-up switcher circuit 81shown in FIG. 16 is omitted and instead the feedback line 83 of thecurrent consumed by the signal level checker circuit 15 is connected tothe start-up corrector circuit 82. The voltage waveforms observed atrelevant points in the switching power supply apparatus during theperiod from the start of its start-up until a shift to steady-stateoperation are the same as in the switching power supply apparatus shownin FIG. 16, and therefore the following description deals only withdifferences in operation.

[0359] In FIG. 17, when, at the time point A3, the internal switch ofthe start-up corrector circuit 82 turns on, the operating current of thesignal level checker circuit 15 and the current though the resistor 34flow through the internal switch of the start-up corrector circuit 82,and thus the signal level checker circuit 15 starts to operate.Moreover, as the result of the internal switch of the start-up correctorcircuit 82 tuning on as described above, the resistor 39 b is connectedbetween the FB terminal T2 of the IC 38 and the negative power supplyline 2.

[0360] During the period from the time point A0 to the time point A3, adiode 88 prevents a current from flowing along the route from thepositive terminal of the capacitor 46 to the operating current supplyline 89 of the signal level checker circuit 15, to the signal levelchecker circuit 15, to the resistor 34, to the resistor 39 b, to theresistor 39 a, to the negative power supply line 2, and to the capacitor46. Thus, during that period, the diode 88 prevents the signal levelchecker circuit 15 from operating.

[0361] The switching power supply apparatus of this embodiment has asimpler circuit configuration than the switching power supply apparatusof the embodiment shown in FIG. 16. However, disadvantageously, theswitching power supply apparatus of this embodiment is susceptible tothe temperature-related drift of the forward voltage drop across thediode 88, and in addition the load current target value at whichswitching between burst switching and continuous switching is performedincreases the temperature-related drift. Thus, this configuration issuitable in applications where the effects of temperature-drift can beignored.

[0362] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe IC 38 is also stopped. This helps reduce the power loss sufferedwhile the switching operation is being stopped, and thus helps reducethe power consumption of the apparatus as a whole.

[0363] Moreover, the start-up corrector circuit 82 so operates that,when the switching power supply apparatus shifts from start-up operationto steady-state operation, the second resistor 39 b is connected inparallel with the first resistor 39 a to reduce the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2.This lowers the potential at the FB terminal T2, and thereby ensuresthat the switching power supply apparatus performs reliable outputvoltage stabilizing control when operating in the steady state.

[0364] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation.

[0365] Fifteenth Embodiment

[0366]FIG. 20 is a circuit diagram of the switching power supplyapparatus of a fifteenth embodiment of the invention. In FIG. 20, suchcircuit components that find their counterparts in FIG. 18 areidentified with the same reference numerals, and their explanations willnot be repeated.

[0367] As shown in FIG. 7, at the start of the start-up of the switchingpower supply apparatus, when, at the time point A3, a current starts toflow through the phototransistor 20 b, and the voltage at the nodebetween the emitter of the phototransistor 20 b and the diode 80increases, this voltage, through the base resistor 87, turns thetransistor 86 on. As the result of the transistor 86 turning on, acurrent flows through the resistor 34 and through the serial circuitcomposed of the resistors 50 and 51. This causes a base current to flowthrough the transistor 47, and thus the signal level checker circuit 15starts to operate.

[0368] During the period up to the time point A3, no current flowthrough the signal level checker circuit 15., This prevents thelengthening of the time required for the charge voltage of the capacitor46 to reach the operation starting voltage of the IC 38, i.e., FA5511.Moreover, as the result of the transistor 86 turning on, the serialcircuit composed of the diode 88 and the resistor 39 b is added betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2.This ensures that the switching power supply apparatus performs reliableoutput voltage stabilizing control when operating in the steady state.

[0369] Incidentally, during the period up to the time point A3, thediode 80 prevents the base currents of the transistors 47 and 86 fromflowing along the route from the positive terminal of the capacitor 46to the resistor 49, to the emitter of the transistor 47, to the base ofthe transistor 47, to the resistor 87, to the base of the transistor 86,to the emitter of the transistor 86, to the negative power supply line2, and to the negative terminal of the capacitor 46. Thus, during thatperiod, the diode 80 prevents the transistors 47 and 86 from beingturned on, and thereby prevents the signal level checker circuit 15 fromoperating and the resistor 39 b from being connected through the diode88 between the FB terminal T2 of the IC 38 and the negative power supplyline 2.

[0370] On the other hand, during the period from the time point A0 tothe time point A3, the diode 88 prevents a current from flowing alongthe path from the positive terminal of the capacitor 46 to the resistor49, to the emitter of the transistor 47, to the base of the transistor47, to the resistor 34, to the resistor 39 b, to the resistor 39 a, andto the negative terminal of the capacitor 46. Thus, during that period,the diode 88 prevents the signal level checker circuit 15 fromoperating.

[0371] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal circuit portions of the IC 38, namely the OSC 106, PWMlogic circuit 105, FB terminal T2, and output buffer 101, is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0372] Moreover, the start-up corrector circuit 82 so operates that,when the switching power supply apparatus shifts from start-up operationto steady-state operation, the second resistor 39 b is connected inparallel with the first resistor 39 a to reduce the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2.This lowers the potential at the FB terminal T2, and thereby ensuresthat the switching power supply apparatus performs reliable outputvoltage stabilizing control when operating in the steady state.

[0373] Moreover, the signal level checker circuit 15, start-up correctorcircuit 82, and CS terminal controller circuit 37 can be realized with asimple circuit configuration, and the switching controller circuit canbe realized with an IC 38, i.e., FA5511. This helps reduce the space ofthe circuit board, and thereby reduce the size and cost of the switchingpower supply apparatus.

[0374] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation.

[0375] Sixteenth Embodiment

[0376]FIG. 21 is a circuit diagram of the switching power supplyapparatus of a sixteenth embodiment of the invention. In FIG. 21, suchcircuit components that find their counterparts in FIGS. 10, 16, 18, and19 are identified with the same reference numerals, and theirexplanations will not be repeated.

[0377] In the switching power supply apparatuses shown in FIGS. 16, 18,19, and 20, in burst switching operation, the period in which switchingoperation is stopped and the period in which switching operation isperformed depend, as described earlier, on the delays in the controlperformed by the output voltage control system. By contrast, in theswitching power supply apparatus of this embodiment shown in FIG. 21,like the switching power supply apparatus shown in FIG. 10, thecomparison reference voltage provided within the signal level checkercircuit 15 a is varied between in the period in which switchingoperation is stopped and in the period in which switching operation isperformed so that, according to how the width of this variation is set,the period in which switching operation is stopped and the period inwhich switching operation is performed can be extended and adjusted.

[0378] Specifically, in this embodiment, to obtain power correspondingto the comparison reference power described earlier in connection withFIGS. 18 and 20, in the voltage division circuit composed of theserially connected resistors 50 and 51, the lower-potential-sideresistor 51 is divided into resistors 51 a and 51 b. The node betweenthe resistors 51 a and 51 b is connected through a diode 59 to thecollector of the transistor 53 provided in the CS terminal controllercircuit 37, and the collector of the transistor 53 is connected througha diode 90 to the CS terminal T8 of the IC 38.

[0379] If the diode 90 is not provided (i.e., if the collector of thetransistor 53 and the cathode of the diode 59 are connected directly tothe CS terminal T8 of the IC 38 without the diode 90 connected inbetween), when the switching power supply apparatus starts to start up,during the period in which the internal switches of the start-upswitcher circuit 81 and the start-up corrector circuit 82 are off, ahigh-level voltage is applied from the positive terminal of thecapacitor 46 through the resistors 50 and 5 la and the diode 59 to theCS terminal T8 of the IC 38. This turns off the output of the IC 38 viaits output terminal T5, and thus makes it impossible for the switchingpower supply apparatus to start up. This problem is overcome by theprovision of the diode 90.

[0380] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal circuit portions of the IC 38, namely the OSC 106, PWMlogic circuit 105, FB terminal T2, and output buffer 101, is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0381] Moreover, the start-up corrector circuit 82 so operates that,when the switching power supply apparatus shifts from start-up operationto steady-state operation, the second resistor 39 b is connected inparallel with the first resistor 39 a to reduce the resistance betweenthe FB terminal T2 of the IC 38 and the negative power supply line 2.This lowers the potential at the FB terminal T2, and thereby ensuresthat the switching power supply apparatus performs reliable outputvoltage stabilizing control when operating in the steady state.

[0382] Moreover, through the operation of the start-up switcher circuit81, when the switching power supply apparatus starts to start up, thestart-up current supplied through the start-up resistor 29 is preventedfrom flowing through the signal level checker circuit 1 Sa and therebylengthening the time required for the charge voltage of the capacitor 46to reach the operation starting voltage of the IC 38, i.e., FA5111.

[0383] Moreover, the signal level checker circuit 15 a, start-upswitcher circuit 81, start-up corrector circuit 82, and CS terminalcontroller circuit 37 can be realized with a simple circuitconfiguration, and the switching controller circuit can be realized withan IC 38, i.e., FA5511. This helps reduce the space of the circuitboard, and thereby reduce the size and cost of the switching powersupply apparatus.

[0384] Moreover, the switching controller circuit (IC 38) is separatefrom the main switching device 5, and therefore, as compared with a casewhere the main switching device is formed integrally in a single package(on a single wafer) along with the switching controller circuit andother components, it is possible to adopt a main switching device havinga low on-state resistance. This helps prevent degradation of powerconversion efficiency in heavy-load operation.

[0385] Seventeenth Embodiment

[0386]FIG. 22 is a circuit diagram of the switching power supplyapparatus of a seventeenth embodiment of the invention. In FIG. 22, suchcircuit components that find their counterparts in FIGS. 11 and 16 areidentified with the same reference numerals, and their explanations willnot be repeated. The switching power supply apparatus of this embodimentshown in FIG. 22 omits the start-up corrector circuit 82 shown in FIG.16, and is instead additionally provided with a current adjuster circuit60 shown in FIG. 11.

[0387] In the switching power supply apparatus of this embodiment shownin FIG. 22, a feedback signal is fed from the phototransistor 20 bthrough the diode 80, the resistor 34, and the current adjuster circuit60 to the FB terminal T2 of the IC 38. The current adjuster circuit 60absorbs from the FB terminal T2 of the IC 38 a current proportional tothe voltage at the node between the diode 80 and the resistor 34.

[0388] Accordingly, when the output voltage of the switching powersupply apparatus is, for example, higher than a predetermined value, theoutput voltage detector circuit 9 increases the voltage at the nodebetween the diode 80 and the resistor 34, and the current adjustercircuit 60 increases, in a manner corresponding to the increase in thatvoltage, the current that it absorbs from the FB terminal T2 of the IC38. This causes the voltage at the FB terminal T2 to decrease.

[0389] As this voltage decreases, the PWM logic circuit 105 (see FIG. 6)provided within the IC 38 feeds, via the output terminal T5 of the IC38, the main switching device 5 with a drive signal of which thehigh-level period is short. This causes the current supplied from thesecondary coil 6 of the transformer 3 through the diode 7 to decrease,and thus the output voltage is so controlled as to decrease.

[0390] On the other hand, when the output voltage of the switching powersupply apparatus is, for example, lower than the predetermined value,the output voltage detector circuit 9 decreases the voltage at the nodebetween the diode 80 and the resistor 34, and the current adjustercircuit 60 decreases, in a manner corresponding to the decrease in thatvoltage, the current that it absorbs from the FB terminal T2 of the IC38. This causes the voltage at the FB terminal T2 to increase.

[0391] As this voltage increases, the PWM logic circuit 105 (see FIG. 6)provided within the IC 38 feeds, via the output terminal T5 of the IC38, the main switching device 5 with a drive signal of which thehigh-level period is long. This causes the current supplied from thesecondary coil 6 of the transformer 3 through the diode 7 to increase,and thus the output voltage is so controlled as to increase.

[0392] When the switching power supply apparatus starts to start up, asdescribed earlier, the start-up current that is supplied through thestart-up resistor 29 flows through the signal level checker circuit 15,and this lengthens the time required for the charge voltage of thecapacitor 46 to reach the operation starting voltage of the IC 38, i.e.,FA5511. To prevent this, here, a start-up switcher circuit 81 isadditionally provided.

[0393] The output current (feedback signal) of the phototransistor 20 bis fed through a diode 80 to the signal level checker circuit 15, andthe start-up switcher circuit 81 checks whether the feedback signal ispresent or not by monitoring the voltage at the node between thephototransistor 20 b and the diode 80.

[0394] The current consumed by the signal level checker circuit 15 (thecurrent consumed including that consumed by the comparison referencepower) is fed thereto from the positive terminal of the capacitor 46 byway of a line 84, and is returned by way of a line 83 through the switchprovided within the start-up switcher circuit 81 to the negativeterminal of the capacitor 46. On the other hand, the current through thephototransistor 20 b is fed thereto from the positive terminal of thecapacitor 46, and is returned through the diode 80, thecurrent-detection resistor 34, and the switch provided within thestart-up switcher circuit 81 to the negative terminal of the capacitor46.

[0395] When the switching power supply apparatus starts to start up, theinternal switch of the start-up switcher circuit 81 is off, and theoutput voltage of the switching power supply apparatus is lower than apredetermined target voltage. Thus, no current is consumed by the signallevel checker circuit 15 (including the comparison reference powerprovided therein) or the phototransistor 20 b. Accordingly, the chargevoltage of the capacitor 46, owing to the start-up current fed theretothrough the start-up resistor 29, quickly rises and reaches theoperation start voltage level of the IC 38, i.e., FA5511.

[0396] In the switching power supply apparatus of this embodiment, thesignal level checker circuit 15 achieves burst switching control byrepeatedly turning on and off the CS terminal controller circuit 37provided in the line by way of which the IC 38, which serves as theswitching controller, is supplied with operating power. Moreover, inburst switching control, while the switching operation of the mainswitching device 5 is being stopped, the supply of operating power tothe principal circuit portions of the IC 38, namely the OSC 106, PWMlogic circuit 105, FB terminal T2, and output buffer 101, is alsostopped. This helps reduce the power loss suffered while the switchingoperation is being stopped, and thus helps reduce the power consumptionof the apparatus as a whole.

[0397] Moreover, when the switching power supply apparatus starts up,the current adjuster circuit 60 so operates as to adjust the current atthe FB terminal T2 of the IC 38. Thus, the PWM control IC makes the mainswitching device perform switching operation with a great on-state duty.This helps reduce start-up time.

[0398] Moreover, through the operation of the start-up switcher circuit81, when the switching power supply apparatus starts to start up, thestart-up current supplied through the start-up resistor 29 is preventedfrom flowing through the signal level checker circuit 15 and therebylengthening the time required for the charge voltage of the capacitor 46to reach the operation starting voltage of the IC 38, i.e., FA5111.

[0399] In the embodiments described hereinbefore, FA5511 manufactured byFuji Electric Co., Ltd. is used as the switching controller. However, itis also possible to use any other IC having equivalent functions torealize similar circuit configurations.

[0400] In the switching power supply apparatus disclosed in JapanesePatent Application Laid-Open No. H10-304658 mentioned as prior art, thestart-up circuit needs to adopt a control device resistant to a highvoltage to shut off a voltage (at the drain of an FET serving as themain switching device) obtained by rectifying and smoothing commerciallydistributed alternating-current power. Disadvantageously, this increasesthe costs of this switching power supply apparatus. To overcome thisdisadvantage, the start-up circuit adopts a structure in which the mainswitching device is formed in a single package along with othercomponents including the control device. However, with the currenttechnology, it is impossible to form a main switching device with a lowon-state resistance in a single package along with such othercomponents. This leads to lower power conversion efficiency when theswitching power supply apparatus is operating in a heavy-load state.

[0401] To solve this problem, in the switching power supply apparatusesof the embodiments described hereinbefore, the switching controller isseparated from the main switching device. This makes it possible to usea main switching device having a low on-state resistance and therebyachieve high power conversion efficiency.

[0402] As described above, according to the present invention, aswitching power supply apparatus uses as a feedback signal the result ofcomparison between the output direct-current voltage and a predeterminedreference voltage, and drives the main switching device by turning onand off, according to the signal level of the feedback signal, thesupply of operating power to a main switching device driving system thatdrives the main switching device. Thus, while the switching operation ofthe main switching device is being stopped in burst switching control,the supply of operating power to the main switching device drivingsystem is also stopped. This helps reduce the power loss suffered whilethe switching operation is being stopped, and thus helps reduce thepower consumption of the apparatus as a whole.

[0403] According to the present invention, a switching power supplyapparatus includes: an output voltage detector that compares the outputdirect-current voltage with a predetermined reference voltage and thatoutputs the result of the comparison as a feedback signal; a switchingcontroller that drives and controls the main switching device accordingto the feedback signal output from the output voltage detector; a signallevel checker that monitors the signal level of the feedback signal andthat outputs an operation control signal for turning on and off theswitching controller according to the monitored signal level; and anoperation/nonoperation switcher that is provided in the line by way ofwhich the switching controller is supplied with operating power and thatturns on and off the switching controller according to the operationcontrol signal from the signal level checker. Thus, burst switchingcontrol is achieved as a result of the signal level checker repeatedlyturning on and off the operation/nonoperation switcher provided in theline by way of which the switching controller is supplied with operatingpower. Moreover, while the switching operation of the main switchingdevice is being stopped in burst switching control, the supply of theoperating power to the switching controller is also stopped. This helpsreduce the power loss suffered while the switching operation is beingstopped, and thus helps reduce the power consumption of the apparatus asa whole.

[0404] According to the present invention, in a switching power supplyapparatus, the signal level of the feedback signal is compared with thesignal level of a previously generated oscillation signal, so that,according to the result of the comparison, the on-state duty of thedrive signal to be fed to the main switching device is determined andswitching between burst switching control and continuous switchingcontrol is performed. Moreover, while the switching operation of themain switching device is being stopped in burst switching control,supply of the operating power for driving the main switching device isstopped. Thus, switching between burst switching and continuousswitching can be performed with high accuracy. Moreover, while theswitching operation of the main switching device is being stopped inburst switching control, the supply of the operating power for drivingthe main switching device is also stopped. This helps reduce the powerloss suffered while the switching operation is being stopped, and thushelps reduce the power consumption of the apparatus as a whole.

What is claimed is:
 1. A switching power supply apparatus having aserial circuit, including a primary coil of a transformer and a mainswitching device, connected between a positive and a negative powersupply line connected to a direct-current power source, the switchingpower supply apparatus outputting a direct-current voltage obtained byrectifying with a rectifier a high-frequency voltage induced in asecondary coil of the transformer by the main switching deviceperforming switching operation, wherein the switching power supplyapparatus uses as a feedback signal a result of comparison between thedirect-current voltage and a predetermined reference voltage, and drivesthe main switching device by turning on and off, according to a signallevel of the feedback signal, supply of operating power to a mainswitching device driving system that drives the main switching device.2. A switching power supply apparatus having a serial circuit, includinga primary coil of a transformer and a main switching device, connectedbetween a positive and a negative power supply line connected to adirect-current power source, the switching power supply apparatusoutputting a direct-current voltage obtained by rectifying with arectifier a high-frequency voltage induced in a secondary coil of thetransformer by the main switching device performing switching operation,wherein the switching power supply apparatus further includes: an outputvoltage detector that compares the direct-current voltage obtainedthrough rectification with a predetermined reference voltage and thatoutputs a result of the comparison as a feedback signal; a switchingcontroller that drives and controls the main switching device accordingto the feedback signal output from the output voltage detector; a signallevel checker that monitors a signal level of the feedback signal andthat outputs an operation control signal for turning on and off theswitching controller according to the monitored signal level; and anoperation/nonoperation switcher that is provided in a line by way ofwhich the switching controller is supplied with operating power and thatturns on and off the switching controller according to the operationcontrol signal from the signal level checker, the switching power supplyapparatus outputting a desired voltage by driving the main switchingdevice with a drive signal from the switching controller that is soturned on and off.
 3. A switching power supply apparatus as claimed inclaim 2, wherein the feedback signal from the output voltage detector istransmitted to the switching controller through a photodiode of aphotocoupler, and the signal level checker monitors the signal level ofthe feedback signal by comparing a current level flowing through aphototransistor of the photocoupler with a reference current level.
 4. Aswitching power supply apparatus as claimed in claim 3, wherein acurrent detection resistor is connected in series with thephototransistor of the photocoupler, and the signal level checker turnson and off the switching controller by feeding the switching controllerwith, as the operation control signal, a signal obtained by comparing avoltage drop across the current detection resistor with a voltage of acurrent level check reference power source.
 5. A switching power supplyapparatus as claimed in claim 3, wherein operating power of the signallevel checker and the phototransistor of the photocoupler is suppliedfrom subsidiary control power extracted from a node between a pluralityof diodes constituting a serial circuit provided in a steady-operationcurrent supply line by way of which a voltage induced in a subsidiarycoil of the transformer is supplied after being rectified with theplurality of diodes.
 6. A switching power supply apparatus as claimed inclaim 2, wherein the operating power of the switching controller issupplied by way of a startup current supply line by way of which astart-up current is supplied from the positive power supply line througha start-up resistor, or by way of a steady-operation current supply lineby way of which a voltage induced in a subsidiary coil of thetransformer is supplied after being rectified with a serial circuitcomposed of a plurality of diodes, and operating power of the signallevel checker is supplied from subsidiary control power extracted from anode between the plurality of diodes.
 7. A switching power supplyapparatus as claimed in claim 2, wherein the switching controller isrealized as a PWM control circuit that outputs, as the drive signal withwhich to drive the main switching device, a pulse signal that ispulse-width-modulated according to a voltage level of the feedbacksignal from the output voltage detector.
 8. A switching power supplyapparatus as claimed in claim 7, wherein used as the PWM control circuitis a PWM control IC that is realized as an integrated circuit chiphaving at least an FB terminal to which a voltage related to thefeedback signal is input and a CS terminal to which a voltage forenabling or disabling an internal circuit is input.
 9. A switching powersupply apparatus as claimed in claim 2, wherein, when the PWM control ICis used as the switching controller, a start-up corrector isadditionally provided to correct start-up of the PWM control IC; a firstresistor is connected between the FB terminal of the PWM control IC andthe negative power supply line; the signal level checker feeds a CSterminal controller, which serves as the operation/nonoperationswitcher, and the FB terminal with the operation control signal and aninverted feedback signal, respectively, according to a result ofchecking of the signal level of the feedback signal; the CS terminalcontroller connects and disconnects the CS terminal of the PWM controlIC to and from the negative power supply line according to the operationcontrol signal; and the start-up corrector connects and disconnects,through a second resistor, the FB terminal to and from the negativepower supply line according to a voltage level of the subsidiary controlpower.
 10. A switching power supply apparatus as claimed in claim 9,wherein the CS terminal controller includes an NPN-type transistorhaving a collector thereof connected to the CS terminal of the PWMcontrol IC, having an emitter thereof connected to the negative powersupply line, and having a base thereof connected to the collector of theother of the transistors included in the signal level checker.
 11. Aswitching power supply apparatus as claimed in claim 9, wherein thestart-up corrector includes: a serial circuit composed of a Zener diodeand a plurality of resistors connected between a line of the subsidiarycontrol power and the negative power supply line; and an NPN-typetransistor having a base thereof connected to a node between theresistors, having a collector thereof connected through the secondresistor to the FB terminal of the PWM control IC, and having an emitterthereof connected to the negative supply power line.
 12. A switchingpower supply apparatus as claimed in claim 9, wherein the signal levelchecker includes, for generation of the reference voltage, voltagedivision resistors, of which a lower-potential-side resistor is dividedinto two resistors, with a node therebetween connected through a diodeto the CS terminal of the PWM control IC.
 13. A switching power supplyapparatus as claimed in claim 9, wherein the switching power supplyapparatus further includes: a capacitor connected between the CSterminal of the PWM control IC and the negative power supply line; and adiode connected between the capacitor and the CS terminal.
 14. Aswitching power supply apparatus as claimed in claim 2, wherein thesignal level checker includes a pair of transistors having emittersthereof connected together to form a comparator, with a base of one ofthe transistors connected to a node between the current detectionresistor and the phototransistor, with a base of the other of thetransistors connected to the current level check reference power source,with a collector of the one of the transistors connected to the FBterminal of the PWM control IC, and with a collector of the other of thetransistors connected to the CS terminal controller.
 15. A switchingpower supply apparatus as claimed in claim 2, wherein, when the PWMcontrol IC is used as the switching controller, the switching powersupply apparatus further includes: a current adjuster connected betweenthe FB terminal of the PWM control IC and the negative power supply lineto adjust a current output from the FB terminal according to the signallevel of the feedback signal; and a CS terminal controller that servesas the operation/nonoperation switcher by connecting and disconnectingthe CS terminal of the PWM control IC to and from the negative powersupply line according to an output signal of the signal level checker.16. A switching power supply apparatus as claimed in claim 15, whereinthe current adjuster includes an NPN-type transistor having a collectorthereof connected to the FB terminal of the PWM control IC, having anemitter thereof connected through a resistor to the negative powersupply line, and having a base thereof connected to a line of thefeedback signal.
 17. A switching power supply apparatus as claimed inclaim 15, wherein the current adjuster includes an NPN-type transistorhaving a collector thereof connected to the FB terminal of the PWMcontrol IC, having an emitter thereof connected through a resistor tothe negative power supply line, and having a base thereof connected to aline of the feedback signal, and in series with the resistor connectedbetween the base of the NPN-type transistor and the negative powersupply line is connected an NPN-type transistor having a collector and abase thereof connected together.
 18. A switching power supply apparatushaving a serial circuit, including a primary coil of a transformer and amain switching device, connected between a positive and a negative powersupply line connected to a direct-current power source, the switchingpower supply apparatus outputting a desired direct-current voltage bycontrolling the main switching device according to a feedback signalobtained as a result of comparison between a direct-current voltageobtained through rectification of a high-frequency voltage induced in asecondary coil of the transformer by the main switching deviceperforming switching operation and a previously set reference voltage,wherein a signal level of the feedback )signal is compared with a signallevel of a previously generated oscillation signal; according to aresult of the comparison, an on-state duty of a drive signal to be fedto the main switching device is determined and switching between burstswitching control and continuous switching control is performed; andwhile switching operation of the main switching device is being stoppedin burst switching control, supply of operating power for driving themain switching device is stopped.
 19. A switching power supply apparatusas claimed in claim 18, wherein burst switching control is achieved byturning on and off supply of operating power to a switching controllerthat drives the main switching device.
 20. A switching power supplyapparatus as claimed in claim 18, wherein, when the PWM control IC isused as the switching controller, a capacitor is connected between an FBterminal of the PWM control IC and an internal power terminal connectedto an internal power supply line.
 21. A switching power supply apparatusas claimed in claim 18, wherein, when the PWM control IC is used as theswitching controller, a serial circuit composed of a capacitor and aresistor is connected between an FB terminal of the PWM control IC andan internal power terminal connected to an internal power supply line.22. A switching power supply apparatus as claimed in claim 2, wherein,when the PWM control IC is used as the switching controller, a start-upcorrector is additionally provided to correct start-up of the PWMcontrol IC; a start-up switcher is additionally provided to turn on andoff supply of operating power to the signal level checker; a firstresistor is connected between the FB terminal of the PWM control IC andthe negative power supply line; the signal level checker feeds a CSterminal controller, which serves as the operation/nonoperationswitcher, and the FB terminal with the operation control signal and aninverted feedback signal, respectively, according to a result ofchecking of the signal level of the feedback signal; the CS terminalcontroller connects and disconnects the CS terminal of the PWM controlIC to and from the negative power supply line according to the operationcontrol signal; the start-up corrector detects whether or not thefeedback signal is present so that, if the feedback signal is present,the start-up corrector connects, through a second resistor, the FBterminal of the PWM control IC to the negative power supply line and, ifnot, the start-up corrector cuts off the second resistor; and thestart-up switcher detects whether or not the feedback signal is presentso that, if the feedback signal is present, the start-up switcher turnson supply of the operating power to the signal level checker and, ifnot, the start-up switcher turns off supply of the operating power tothe signal level checker.
 23. A switching power supply apparatus asclaimed in claim 22, wherein the start-up switcher includes an NPN-typetransistor having a collector thereof connected to a node between acurrent detection resistor connected to a line of the feed back signaland an internal reference voltage line of the signal level checker,having a base thereof connected to the phototransistor, and having anemitter thereof connected to the negative power supply line.
 24. Aswitching power supply apparatus as claimed in claim 22, wherein thestart-up corrector includes an NPN-type transistor having a collectorthereof connected through the second resistor to the FB terminal of thePWM control IC, having a base thereof connected through a resistor tothe phototransistor, and having an emitter thereof connected to thenegative power supply line.
 25. A switching power supply apparatus asclaimed in claim 2, wherein, when the PWM control IC is used as theswitching controller, a start-up corrector is additionally provided tocorrect start-up of the PWM control IC; a first resistor is connectedbetween the FB terminal of the PWM control IC and the negative powersupply line; the signal level checker feeds a CS terminal controller,which serves as the operation/nonoperation switcher, and the FB terminalwith the operation control signal and an inverted feedback signal,respectively, according to a result of checking of the signal level ofthe feedback signal; the CS terminal controller connects and disconnectsthe CS terminal of the PWM control IC to and from the negative powersupply line according to the operation control signal; and the start-upcorrector detects whether or not the feedback signal is present so that,if the feedback signal is present, the start-up corrector connects,through a diode and the second resistor, the FB terminal of the PWMcontrol IC to the negative power source line and turns on supply ofoperating power to the signal level checker and, if not, the start-upcorrector cuts off the diode and the second resistor and turns offsupply of the operating power to the signal level checker.
 26. Aswitching power supply apparatus as claimed in claim 25, wherein thestart-up corrector includes an NPN-type transistor having a collectorthereof connected through the diode and the second resistor to the FBterminal of the PWM control IC, having a base thereof connected througha resistor to the phototransistor, and having an emitter thereofconnected to the negative power supply line.
 27. A switching powersupply apparatus as claimed in claim 25, wherein the signal levelchecker includes, for generation of the reference voltage, voltagedivision resistors, of which a lower-potential-side resistor is dividedinto two resistors, with a node therebetween connected through a diodeto the CS terminal controller, and the CS terminal controller isconnected through another diode to the CS terminal of the PWM controlIC.
 28. A switching power supply apparatus as claimed in claim 2,wherein, when the PWM control IC is used as the switching controller, astart-up switcher is additionally provided to turn on and off supply ofoperating power to the signal level checker; a current adjuster isadditionally provided that is connected between the FB terminal of thePWM control IC and the negative power supply line to adjust a currentoutput from the FB terminal according to the signal level of thefeedback signal; the signal level checker feeds a CS terminalcontroller, which serves as the operation/nonoperation switcher, withthe operation control signal according to a result of checking of thesignal level of the feedback signal; the CS terminal controller connectsand disconnects the CS terminal of the PWM control IC to and from thenegative power supply line according to the operation control signal;and the start-up switcher detects whether or not the feedback signal ispresent so that, if the feedback signal is present, the start-upswitcher turns on supply of operating power to the signal level checkerand, if not, the start-up switcher turns off supply of operating powerto the signal level checker.